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UBC Theses and Dissertations

A seismic refraction survey along the southern Rocky Mountain Trench Bennett, Geoffrey Taylor 1973

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A SEISMIC REFRACTION SURVEY ALONG THE SOUTHERN ROCKY MOUNTAIN TRENCH by GEOFFREY TAYLOR BENNETT B.Eng., Royal M i l i t a r y College of Canada, 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of GEOPHYSICS and ASTRONOMY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1973 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geophysics & Astronomy The University of British Columbia Vancouver 8, Canada Date October 13, 1973 i ABSTRACT Deducing the structure and o r i g i n of the Rocky Mountain Trench has proven to be a d i f f i c u l t problem i n the past. To under-stand this feature more f u l l y and to obtain information about the entire c r u s t a l section, an unreversed seismic refraction p r o f i l e has been recorded i n the southern Rocky Mountain Trench from 50°N to 53°N. Using blasts from two open p i t coal mines, forty-four useful recordings were obtained over a distance of 540 km. Three components of short period ground motion were recorded by tape recording systems; the v e r t i c a l component was also recorded by elements of the Mica array. Careful attention to amplitude scale factors results i n the formation of a record section i n which the energy pattern varies uniformly along the p r o f i l e . A geometric ray theory in t e r p r e t a t i o n involving Weichert-Herglotz integration of p-delta curves i s used to obtain a velocity-depth structure. Approximate synthetic seismograms are then calculated using modified ray theory. Refractors with apparent P-wave v e l o c i t i e s of 6.5 - 6.6 km/s and 8.22 ± 0.04 km/s are interpreted as the surface of the Precambrian basement and the Moho discontinuity, respectively. A v e l o c i t y gradient i s present i n the lower cr u s t a l section. The depth to basement beneath the western Rocky Mountains at 50°30'N i s calculated to be 6.5 ± 1 km. Near Radium, a s i g n i f i c a n t anomaly i n the seismic data i s best interpreted as a northeasterly-trending normal f a u l t with a downthrow of 5.6 ± 1 km to the northwest. The directions are inferred from gravity and magnetic trends i n the region. A l t e r n a t i v e l y , the anomaly could represent a disappearance of the basement surface west of the east w a l l of the Trench. An anomalously thick c r u s t a l section i s inferred from the data. A preferred model gives a depth of 51 ± 2 km southeast of Radium and 58 ± 2 km to the northwest. Study of a converted phase leads to the conclusion that there may be a discontinuity on the Moho o surface beneath the Trench near 52 N. Analysis of a r r i v a l s shortly a f t e r the P phase i s consistent with the interpretation of a low n ve l o c i t y zone, approximately 7 km thick, 8 km beneath the Moho. i i TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS i i LIST OF TABLES iv LIST OF FIGURES v ACKNOWLEDGEMENTS v i i INTRODUCTION 1 THE ROCKY MOUNTAIN TRENCH 3 General description 3 The controversy 5 PREVIOUS GEOPHYSICAL STUDIES IN THE REGION OF THE TRENCH 7 Seismic refraction and reflection profiles 7 Gravity surveys 9 Magnetic surveys 9 COLLECTION OF DATA 11 Seismic instrumentation 11 Field methods 14' ' PRELIMINARY DATA ANALYSIS 19 Digitization 19 Filtering 19 Formation of record sections 24 INTERPRETATION 27 General features of the record section 27 Interpretation technique 31 Structure based on arrivals from 0 to 150 km 32 Low velocity zones 35 Fault models 40 Late arrivals beyond 400 km 48 DISCUSSION OF THE RESULTS 51 The basement near 50°30'N, 115°W 51 Low velocity zones 52 The basement fault 53 Possible significance of the fault with respect to 56 | Trench origin , Crustal thickness 57 : Converted S phase 59 i i i Page CONCLUSION 60 REFERENCES 62 APPENDIX Corrections for HRGLTZ Fault Models 66 1. Correction to P intercept time for fault 66 g on basement 2.0 General correction to P intercept time 67 n 2.1 Correction for basement fault only 68 2.2 Correction for fault with constant throw 69 throughout the crustal section 2.3 Correction for fault on the Moho with throw 69 calculated from the pre-fault P intercept i v LIST OF TABLES Table Page I Shot point data 16 V LIST OF FIGURES Figure Page 1 The southern Rocky Mountain Trench 2 2a Shot point recording system 12 2b Typical 3-component portable seismic recording 12 system 2c Mica Creek telemetered seismic array 13 3 Typical v e l o c i t y s e n s i t i v i t y curve 15 4a Comparison of power spectra for p r e - a r r i v a l 20 noise and for the seismic s i g n a l at A3 4b Comparison of power spectra for p r e - a r r i v a l 21 noise and for the seismic s i g n a l at C6 5a Seismic signal at A3 before and after application 22 of Butterworth bandpass f i l t e r 5b Seismic s i g n a l at C6 before and after application 23 of Butterworth bandpass f i l t e r 6 Normalized record section with no correction 28 for geometrical spreading 7 Normalized record section with distance squared 29 correction for geometrical spreading 8 Velocity-depth structure and p-delta curves 33 for two basement models 9 Synthetic seismograms for two basement models 34 10 Velocity-depth structure and p-delta curve for 36 the low v e l o c i t y zones interpretation 11a F i t of the traveltimes to the record section for 37 the low v e l o c i t y zones interpretation l i b Synthetic seismograms for the low v e l o c i t y zones 38 int e r p r e t a t i o n 12a Velocity-depth structure and p-delta curves f o r 42 a model with a basement f a u l t and an unfaulted Moho 12b Traveltime curves f o r the velocity-depth structure 43 i n Figure 12a 13a Velocity-depth structure and p-delta curves for 44 a model with constant f a u l t throw throughout the c r u s t a l section 13b Traveltime curves for the velocity-depth structure 45 i n Figure 13a 14a Velocity-depth structure and p-delta curves f o r 46 a model which f i t s ah assumed up-fault P^ intercept v i Figure _ Page 14b Traveltime curves for the velocity-depth structure 47 i n Figure 14a 15 P a r t i c l e motion diagram of the late A12 49 large amplitude phase 16 Bouguer gravity anomaly map of the southern 54 Rocky Mountains 17 Residual t o t a l magnetic f i e l d map for southern 55 ALberta and southeastern B r i t i s h Columbia v i i ACKNOWLEDGEMENTS / Throughout this project, the f i e l d e f f o r t s and l i b e r a l guidance of Drs. R.M. Clowes and R.M. E l l i s have been greatly appreciated. Mr. R.D. Meldrum provided invaluable and patient assistance with the instrumentation and d i g i t i z a t i o n . Without the advice of Dr. R.A. Wiggins and the use of his Weichert-Herglotz program, the interpretation would have been much more d i f f i c u l t . The stalwart help from Messrs. P. Somerville, G. Pareja, K. Roxburgh and J. Peters i n the f i e l d i s g r a t e f u l l y acknowledged. Special thanks must go to the personnel of Kaiser Resources and Fording Coal Ltd. who generously permitted the shot point recordings and who supplied the location data. The assistance of the Earth Physics Branch, Ottawa i n determining several s i t e locations i s much appreciated. To my wife, Wendy, I tender my most sincere thanks, not only for providing moral support, but for drafting most of the diagrams as w e l l . This work was supported by grants from the Department of Energy, Mines and Resources, the B.C. Department of Mines, the National Research Council (Grant A-2617), the University of B.C., and by a personal scholarship to the writer from the National Research Council. INTRODUCTION The Rocky Mountain Trench i s a long, narrow, intermontane valley bordering the west flank of the Canadian Rocky Mountains. As a subject of geologic controversy i t i s almost without peer, and much speculation has been directed towards i t s o r i g i n and underlying structure. An understanding of i t s deep structure would not only be important i n i t s e l f , but would also y i e l d valuable information about the formation of the entire eastern Cordilleran system. Relatively few geophysical p r o f i l e s have shed any l i g h t on deep c r u s t a l structure i n this region. Seismic r e f r a c t i o n studies have generally concentrated on the western C o r d i l l e r a or the In t e r i o r Platform of Alberta; analyses of gravity data across the Trench have o only pertained to the region south of 51 N; magnetic data have mainly been useful i n determining the l a t e r a l structure only. Subsequently, i t has been discovered that large b l a s t s from open p i t coal mines near Sparwood (Figure 1) are regularly recorded on standard seismic stations. Using these blasts as good inexpensive sources of seismic energy, i t was decided to run an unreversed seismic re f r a c t i o n p r o f i l e northwest along the southern Rocky Mountain Trench. Geophysicists from the University of B.C. performed the necessary f i e l d work during the summers of 19 72 and 19 73, recording a detailed p r o f i l e along the Trench from 50°N to 53°N. As the analysis i n this thesis w i l l show, the seismic data have led to new conclusions regarding c r u s t a l structure beneath the Rocky Mountain Trench. 2 Figure 1. The Southern Rocky Mountain Trench Geologic setting and location of 19 72-73 seismic recording s i t e s (inset after Douglas (1970)) Explanation of symbols: portable seismic recording system (Kaiser shot only) portable seismic recording system (Kaiser and Fording shots) Mica array component shot point town physiographic outline of Trench p r o v i n c i a l boundary ^~—*— r i v e r A ® 2 a 3 THE ROCKY MOUNTAIN TRENCH General description During the l a t e 1850's, P a l l i s e r and Hector became the f i r s t men to remark openly on the "unbroken continuity" of the Trench, although t h e i r outlook was biased by the necessity of finding an easy route to the Thompson and Fraser Rivers (Schofield, 1921). In traversing the Trench near the in t e r n a t i o n a l boundary, Dawson (1886) became the f i r s t geologist to recognize the "Columbia-Kootanie Valley" as "an orographic feature of the f i r s t importance." He was also the originator of what was to become one of the favourite preoccupations of Cordilleran geologists - speculation on the o r i g i n of the Rocky Mountain Trench. As defined and named by Daly (1912), the Trench i s a long, narrow, intermontane depression occupied by two or more streams alternately draining the depression i n opposite directions. The Trench i s long indeed, stretching over 1600 km from Montana to northern B r i t i s h Columbia (Figure 1). Its average s t r i k e i s N 33° W, interrupted o only at 55 N where the physiographic characteristics of the Trench fade into the Nechako plateau. In the south i t s trend i s s l i g h t l y sinuous, especially south of 50°N where the Trench widens to about 32 km (20 miles) as compared with an average width of 3 - 16 km (2 - 10 miles). I t contains eight major r i v e r s , separated by almost imperceptible gradients. Curiously, the l o c a l width of the Trench bears no r e l a t i o n to the s i z e of the r i v e r occupying i t . The elevation of the Trench f l o o r varies only s l i g h t l y from 2000 to 2700 feet above sea l e v e l along i t s entire length. Almost everywhere i t i s covered by a thick blanket of detritus - l o c a l l y 1500 m (5000 feet) thick southwest of Sparwood (Lamb and Smith, 1962). On either side the walls r i s e to 900 - 1800 m (3000 - 6000 feet) above the f l o o r , the eastern side generally being more precipitous than the more i r r e g u l a r western side. These physical c h a r a c t e r i s t i c s are s t r i k i n g enough, but the Trench i s perhaps best known for i t s appearance as a l a t e r a l boundary between two r a d i c a l l y d i f f e r e n t terranes to the southwest; and northeast. Physiographically i t marks the dividing l i n e between the Rocky Mountains to the east and the P u r c e l l , S e l k i r k , Monashee, 4 Cariboo, Omineca and Cassiar mountains to the west. Although the Trench i s p a r a l l e l to the regional trend of the Rocky Mountain Thrust B e l t , i n d i v i d u a l ranges on both sides are s l i g h t l y truncated (North and Henderson, 1954). Geologically, the contrast i s even more apparent. The Rocky Mountains have been formed exclusively from sedimentary miogeosynclinal s t r a t a , predominantly carbonates and e l a s t i c s . These layers have been thrust to the northeast res u l t i n g i n the well-known imbricate structure of this mountain b e l t (Price and Mountjoy, 1970; Bally et a l , 1966). To many writers (Holland, 1959), the lack of intrusions and volcanic a c t i v i t y i n the Trench and to the east i s remarkable. In marked contrast, to the southwest l i e s the Omineca C r y s t a l l i n e B e l t , a region of highly metamorphosed i n t r u s i v e g r a n i t i c rocks. Deformation and buoyant upwelling were i n i t i a t e d i n this b e l t during the l a t e Jurassic, causing g r a v i t a t i o n a l spreading to the east and subsequent formation of the Rocky Mountain thrust sheets (Price and Mountjoy, 1970). The boundary zone occupied by the Trench must continue to greater depths since geophysical properties also show marked variations across i t . A study of magnetic anomalies over western Canada by Haines et a l (19 71) shows two d i s t i n c t zones i n t h i s area. East of the Trench i s the "Shield Zone", characterized by broad high amplitude anomalies, s t r i k i n g north to northeast. To the west l i e s the "Cordilleran Zone" showing long, narrow, northwest-trending anomalies. Geomagnetic depth-sounding p r o f i l e s have consistently found a t r a n s i t i o n from low I to high I west to east across the Trench, where I i s the v e r t i c a l / h o r i -zontal amplitude r a t i o of the l o c a l geomagnetic f i e l d (Caner et al,^1971; Dragert, 1973). Using gravity data and noting the absence of a high ve l o c i t y layer i n the lower crust under the western C o r d i l l e r a , Stacey (19 73) has postulated a decrease i n the density of the crust and upper mantle to the west of the Trench. Further reference to some of these studies w i l l be made l a t e r . Perhaps such geophysical anomalies point to a r e l a t i v e l y simple and umique o r i g i n f o r the Trench, but detailed geological studies along i t s walls do not support such a conclusion. No "standard" theory of erosion or s t r u c t u r a l control has found wide acceptance along the entire length of the Trench, even though each idea i s strongly supported by evidence i n d i f f e r e n t segments. Yet the unique continuity 5 and length of the Trench beg for a unifying i n t e r p r e t a t i o n , and i t i s this fact which has spawned a fascinating l i t e r a t u r e i n Cordilleran studies. The controversy Dawson (1886) regarded the Trench as a s t r i c t l y erosional feature which formed subsequent to the u p l i f t of the Rocky Mountains. Daly (1912) e n t h u s i a s t i c a l l y c a l l e d the Trench "unique among a l l the mountain features of the globe for i t s remarkable persistence," and attributed i t s o r i g i n to "a l i n e of d i s l o c a t i o n , " perhaps a graben. Several years l a t e r , Daly (1915) changed his opinion and offered an alternative explanation as a zone of transcurrent f a u l t i n g . His reason was that the Trench does not have a t y p i c a l symmetric graben structure. Schofield (1921) returned to an erosional theory and described at great length how ancient t r i b u t a r i e s carved t h e i r paths through the Trench to produce such odd features as the Big Bends of the Fraser and Columbia Rivers. Even so, his theory i s contingent upon t i l t i n g and down-dropping of f a u l t blocks within the Trench. South of Golden, Shepard (1922) postulated thrust f a u l t control and erosion along a zone of weakness caused by the intersection of a large number of f a u l t planes. Finding westerly-dipping folds to the west and easterly-dipping folds to the east, he also concluded that the Trench could be a depression caused by this i n - f o l d i n g pattern. Later, Shepard (1926) discounted Daly's graben structure completely and proposed, of a l l things, a horst. Admitting this was unusual, he r e l i e d on long-continued erosion to change the u p l i f t e d horst into a depression. Walker (1926) and Evans (1933) also postulated thrust f a u l t control along which subsequent erosion could e a s i l y work, but discounted a l l theories of transcurrent f a u l t i n g and graben or horst structure. Without much elaboration, Eardley (1947) linked the Trench to s i m i l a r features i n the United States to produce a belt 3200 km (2000 miles) long, presumably a r i f t zone. North and Henderson (1954) synthesized previous theories, then devised a clever i n t e r p r e t a t i o n i n which the co n t r o l l i n g faults originated as transcurrent f a u l t s but were l a t e r converted into thrusts. And j u s t when the question appeared to be s e t t l e d , Crickmay (1964) vehemently refuted these f a u l t theories, postulating an o r i g i n by the mere headward erosion of streams 6 i n "a region i n v i t i n g to trench-making." The importance of erosion i n forming the present physiographic outline i s undeniable (Simony et a l (19 72) found evidence for g l a c i a t i o n by an i c e sheet 600 - 900 m (2000 - 3000 feet) t h i c k ) , but the lack of some form of s t r u c t u r a l control i s dubious, to say the l e a s t . Leech (1959) suggested that the southern part of the Trench has been formed by Cenozoic block f a u l t i n g . This idea was substantiated by Garland et a l (1961), Thompson (1962), and J. Claig (oral communica-t i o n , 1973). In addition, Gabrielse (1972) hypothesized block f a u l t i n g i n the northern Trench following a period of transcurrent movement. Detailed seismic r e f l e c t i o n studies by Ba l l y et a l (1966) favour a compatible idea that the Trench was formed after the main thrusting phase of the Rockies by normal f a u l t i n g along pre-existing thrust plane surfaces. Dahlstrom (1970) and Price and Mountjoy (1970) concur that low angle normal f a u l t i n g could have occurred during periods of relaxation following the compressive surges. Mudge (19 70) found that a s i m i l a r interpretation i n northwest Montana f i t t e d the obser-vations, although he hypothesized that the Trench i s a "tear-away" zone behind a region of gravity thrusting i n the Rockies. As Crickmay (1964) has stated, "...the existence of the Rocky Mountain Trench i s an enigma, a d i f f i c u l t and persistent puzzle i n geology....The o r i g i n of such valleys i s thus a question that has f a i l e d the best mental powers that have so f a r investigated i t . " Although no f i n a l d e f i n i t i v e solution to this enigma has appeared (nor i s i t l i k e l y t o ) , there remains one theory which i s remarkably persistent throughout the l i t e r a t u r e : "...that the Trench may mark an o l d , deep fracture zone that has reasserted i t s e l f through the allocthonous veneer" (Leech, 1965). This concept w i l l be referred to l a t e r i n discussing the significance of the Rocky Mountain Trench seismic data. 7 PREVIOUS GEOPHYSICAL STUDIES IN THE REGION OF THE TRENCH Seismic r e f r a c t i o n and r e f l e c t i o n p r o f i l e s Within the Trench i t s e l f , only a very l i m i t e d amount of previous seismic work has been carried out. Lamb and Smith (1962) ran a series of shallow reversed r e f r a c t i o n p r o f i l e s along and across the Trench at 49°30'N. A continuous refra c t o r with a P-wave ve l o c i t y of 5.2 - 5.5 km/s (17,000 - 18,000 ft / s ) was mapped across the Trench and was interpreted as the Precambrian P u r c e l l surface. This surface i s exposed at 2500 - 3000 feet above sea l e v e l on the Trench margins and dips to 2000 feet below sea l e v e l i n the centre. I t i s overlain by Cenozoic sediments with a P-wave ve l o c i t y of 3.3 km/s (11,000 f t / s ) and a thin "loose" layer i n the centre of the Trench with a v e l o c i t y of 2.5 km/s (7500 f t / s ) . B a l l y et a l (1966) observed seismic ref l e c t i o n s across the Trench near 49°15'N. Strong, "near-basement" a r r i v a l s beneath the Rocky Mountains were interpreted as re f l e c t i o n s from just above the westward extension of the Precambrian s h i e l d . The Shield, comprised of metamorphic and igneous rocks and referred to as the c r y s t a l l i n e "basement", appears to dip gently westward to the east w a l l of the Trench. Across the Trench, the data was noisy and showed no coherent r e f l e c t i o n s , but the extension of the basement r e f l e c t i o n could be observed again beneath the P u r c e l l mountains for a short distance on the western side. Despite the poor data, Bally et a l (1966) assumed that the basement does ex i s t beneath the Trench at t h i s l a t i t u d e at a depth of 8.5 ± 0.3 km (28,000 ± 1000 f t ) with an over-l y i n g layer of v e l o c i t y 5.2 - 5.5 km/s. To the east, seismic r e f r a c t i o n studies by Cumming and Kanasewich (1966) have been extended and re-analysed by Chandra and Cumming (1972) to delineate the deep c r u s t a l structure beneath southern and central Alberta. Cumming and Kanasewich (1966) gave a depth to the Moho of 46 km beneath the Trench near 50°30'N, although how this figure was obtained i s unclear. The more recent work by Chandra and Cumming (1972) has interpreted a p a r t i a l l y reversed r e f r a c t i o n p r o f i l e along 50°30'N from 118°W to 108°W. An abrupt discontinuity i n seismic v e l o c i t i e s and refractor depths has been postulated beneath the Front Ranges of the Rocky Mountains. A constraint used i n the interpretation of this section was the work of White et a l (1968) i n the Intermontane 8 Belt (Columbian Zwischengebirge i n Figure 1), since that section of the p r o f i l e west of the Front Ranges was unreversed. The upper c r u s t a l v e l o c i t i e s found by White et a l (1968) were assumed to extend to the eastern margin of the Rocky Mountains. Integration with observed Bouguer gravity anomalies has allowed a tentative velocity-depth model beneath the Trench of 6.00 km/s from 0 - 2 0 km, 6.50 km/s from 20 - 31 km, 7.15 km/s from 31 - 49 km, underlain by the upper mantle with P-wave ve l o c i t y 8.20 km/s. I t must be emphasized that this structure, except for the P^ v e l o c i t y , has been deduced by extrapolation of the eastern p r o f i l e s to f i t data recorded i n central B r i t i s h Columbia. Data from seismic refr a c t i o n surveys i n B r i t i s h Columbia up to 1966 were synthesized by White et a l (1968). Signals from the 1958 Ripple Rock explosion i n the S t r a i t of Georgia near Vancouver were recorded throughout eastern B.C. along l a t i t u d e 51°N. From t h i s unreversed p r o f i l e , the data indicated a P n v e l o c i t y of 7.76 km/s and an average c r u s t a l thickness of 31 km i n the eastern C o r d i l l e r a . Subsequent northwest-striking p r o f i l e s i n the Intermontane Belt showed a uniform upper cru s t a l P-wave ve l o c i t y of 6.1 km/s with the Moho discontinuity dipping gently eastward at a rate which would produce a 45 km cru s t a l thickness immediately east of the Rockies. However, this figure was extrapolated from a p r o f i l e i n southwestern B.C. and i s probably an ov e r - s i m p l i f i c a t i o n . Forsyth (1973) found a s i g n i f i c a n t 800 km wavelength on the Moho which shows a rather strong easterly dip i n the Omineca C r y s t a l l i n e B e l t . Using a reversed p r o f i l e i n the v i c i n i t y of 53°N, 122°W, the Moho was observed to dip 10° to the east with an average P v e l o c i t y i n the area of 8.06 km/s. One model for n this p r o f i l e shows an upper c r u s t a l P-wave ve l o c i t y of 5.6 km/s to a depth of 3.5 km, a 6.2 km/s layer with a gradient to 30 km and a 7.5 km/s layer with a gradient to the Moho at 45 km. An alternate model with no 7.5 km/s layer gives a cr u s t a l thickness of 35 km, and was preferred since the evidence for this deep cru s t a l layer i s marginal. Two long range unreversed p r o f i l e s were run i n a southeasterly d i r e c t i o n from Greenbush Lake (51°N, 118°W) i n 1969 (Hales and Nation, 1973). The more westerly p r o f i l e travels through the Omineca C r y s t a l l i n e Belt into the U.S. and gives a model of 6.0 km/s to 22.6 km depth and 6.41 km/s to the Moho at 37.5 km depth. The ? n v e l o c i t y i s 8.04 km/s. The more easterly p r o f i l e , which crosses the Trench i n northern Montana, 9 provides P^ information only (8.04 km/s) but the P^ branch intercept i s the same for both p r o f i l e s (7.87 sec). Gravity surveys A small-scale survey by Garland et a l (1961) found three deep basins f i l l e d with unconsolidated material i n the Trench south of 49°30'N. This discovery lent strong evidence to a theory of block f a u l t i n g i n the Trench at this l a t i t u d e . On a much larger scale, Stacey (1973) has interpreted a gravity p r o f i l e across the C o r d i l l e r a which includes the Trench from 49°N to 51°N. A gravity minimum of -200 mgal occurs over the northern part of the P u r c e l l Mountains and the adjacent Trench. In conjunction with the rest of the data, this i s interpreted as a decrease i n the density of the crust and upper mantle west of the Trench, with an expected c r u s t a l thickness approaching 60 km beneath the Rocky Mountains. However, i f the crust i s shown to be very much thinner than 60 km, thi s density change must begin further east below the Rocky Mountains themselves. Magnetic surveys Combining magnetotelluric and geomagnetic depth-sounding data for the C o r d i l l e r a , Caner et a l (1971) concluded that the C o r d i l l e r a west of the Trench i s underlain by a highly conductive layer s t a r t i n g at a depth of 10 - 15 km with i n d e f i n i t e thickness of about 20 - 40 km. The c r u s t a l layer has a sharply defined eastern margin where the I values begin to increase to the east. This t r a n s i t i o n l i e s a few kilometers to the east of McBride where i t s t r i k e s N 30°-35° W, p a r a l l e l to the Trench. I t continues as far south as Valemount and probably further to the v i c i n i t y of 51°30'N after which i t swings southward i n t o an almost north-south s t r i k e . Interpreting a geomagnetic depth-sounding p r o f i l e across the Trench near Golden, Dragert (19 73) concluded that the highly conductive zone i n the western region i s probably at, or dips to a depth of, about 40 - 50 km beneath the Rocky Mountain Trench t r a n s i t i o n region. To agree with Caner et a l (1971) , this model requires an eastward dip under the Rocky Mountain area of approximately 6°-ll°. 10 Most of these geophysical findings are d i r e c t l y relevant to an in t e r p r e t a t i o n of the Rocky Mountain Trench seismic data. In the l i g h t of this l a t e s t evidence, a number of these p r o f i l e s w i l l be discussed further i n a subsequent section. 11 COLLECTION OF DATA. Seismic instrumentation Recording systems used i n the Rocky Mountain Trench project are i l l u s t r a t e d schematically i n Figures 2a, 2b and 2c. The shot point system was very simple and consisted of a two-channel chart recording of the WWVB time signal and the output of a geophone planted near the bla s t . Three portable seismic recording systems, designated A, B and C, were i n use throughout the project. Each recorded three components of short period ground motion onto FM tape (Figure 2b). Both low and high l e v e l amplifier outputs were recorded so that amplitude information would be preserved i n the event of a si g n a l voltage overload. Also recorded on tape were one or two time s i g n a l s , depending upon the quality of reception. The WWV time s i g n a l was used to trigger.a timing unit which then sent minute and second marks to tape; output from the WWVB receiver was recorded d i r e c t l y . The WWVB signal was generally preferred when obtainable, i n order to improve the accuracy of traveltime calculations. One annoying problem throughout the project was the occurrence of 6 Hz noise on the amplifier.channels of system C (Figure 5b). Eventually the problem was traced to induction effects of the timing unit on the amplifier inputs, but was not s a t i s f a c t o r i l y resolved u n t i l near the end of the project. Less serious electronic noise occurred sporadically i n systems A and B, sometimes necessitating the use of a th i r d battery. The Mica telemetered seismic array was operating by the f a l l of 1972 i n the configuration shown i n Figure 2c. I t should be noted that the data obtained from this array were equivalent i n most respects to that recorded by the portable systems. Tape recorder s p e c i f i c a t i o n s , amplifier f i l t e r cutoffs and timing were the same, although the resonant frequencies of the Mica seismometers were s l i g h t l y higher at 1.63 Hz, and only the v e r t i c a l component of ground motion was recorded. A detailed description of this array has been given elsewhere ( E l l i s and Russell, 1972) and w i l l not be repeated here. Four of the seismic s i t e s are located on peaks above the Rocky Mountain Trench (Mount Thompson (THO), Mount Dainard (DAI), Mount Cummins (CUM), and Tabernacle Mountain (TAB): see Figure 1); one other s i t e (MCC) i s located near the recording s t a t i o n 12 0 1000' cable exploration geophone xl-xlO gain W W V B Brush 2-channel chart recorder 125 mm/sec when possible Figure 2a. Shot point recording system © v e r t i c a l A * * ^ high r a d i a l transverse Willmore Mk I I seismometers C H R 0 N W W V Geotech AS-330 amplifiers T U W W V B T A P E Geotech or PI 7-channel FM tape recorder 40% deviation =2.82 vo l t tape speed = 15/160.ips Figure 2b.. Typical 3-coraponent portable seismic recording system Seismometer natural frequency adjusted to 1 hz. Amplifier low output i s attenuated from 70 db i n 6 db steps. High output separation i s 18,24, or 30 db above low output. F i l t e r cutoffs set at 0.75-12.5 hz. UBC timing unit outputs second and minute marks, e l e c t r o n i c a l l y triggered from WW receiver and controlled by 60 hz output of chronometer. T H O (^y D A I Q c U M T A B M C C Willmore Mk I seismometers ( v e r t i c a l only) 0 1 V 1 T. I Geotech 42.21 telemetry amplifiers Geotech EA-310 amplifiers D Geotech 46.11 FM discriminators W W V B Figure 2c. Mica Creek telemetered seismic array Seismometer natural frequency adjusted to 1.63 Hz. Telemetry amplifiers have 100 db maximum gain; post-amplifiers raise sign by an additional 0,6, or 12 db. F i l t e r cutoffs approximately 1-12.5 Hz. Geotech or PI 7-channel FM tape recorder; 40% deviation =2.82 v o l t s ; tape speed = 15/160 i p s . 14 at Mica Creek and i s operated by the Earth Physics Branch. However, i t s i n f e r i o r signal-to-noise r a t i o made i t of l i t t l e use to this.project. Five shots recorded by this array i n February and June, 1973 were u t i l i z e d i n this study. Following the 1972 f i e l d season the systems were calibrated i n the same configuration as the operating mode, except that the tape recorder was disconnected. The c a l i b r a t i o n technique involved the use of the Maxwell bridge and current source analysis of K o l l a r and Russell (1966). Graphs of acceleration, v e l o c i t y and displacement s e n s i t i v i t y were drawn for each of the nine seismometer/amplifier systems; a t y p i c a l v e l o c i t y s e n s i t i v i t y curve as used i n l a t e r calculations of scale factors i s shown i n Figure 3. The tape recorder response was e s s e n t i a l l y f l a t up to 17 Hz and thus did not affect the t o t a l system response i n the region of in t e r e s t . Following the 1973 f i e l d season the same systems were "spot" calibrated and were found to have deviated by less than 1%. Calibrations of the Mica array were performed by R.D. Meldrum before the system was i n s t a l l e d , and a computer program was used to determine the t o t a l system response. In the f i e l d , two types of transient calibrations were performed on systems A, B and C as an operational check, but were not used i n subsequent analysis. The "K" test described the seismometer response to a voltage step input, while the "S" test described the response of the complete seismometer/amplifier system. F i e l d methods The normal procedure i n the f i e l d was to have one member of the party record the time of detonation at the shot point, while three portable systems were taken to predetermined locations along the Trench to the northwest. Horizontal seismometers were aligned i n r a d i a l and transverse directions to the shot. Communication between shot and receiver was by radio telephone, thus greatly eliminating problems caused by shot cancellations, delays, equipment malfunctions, etc. Large blasts were recorded from the Kaiser Resources open p i t coal mine near Sparwood and the Fording Coal Ltd. open p i t operations 50 km to the north. As i s apparent from Figure 1, both shot points are approximately on s t r i k e with the Trench, with recorded seismic energy actually entering the Trench zone i n the v i c i n i t y of Radium. An unexpected feature of these blasts i s that, despite the large charge size (see Table I ) , 15 1 1 1 1 I I I 1 1 1 1 1 1 1 1 1 1 1 1 - -/ llll 1 1 - / -/ : / -— / 1 1 1 1 1 1 1 1 1 1 l l l l 10 10 10 10 0.2 1 3 10 F R E Q U E N C Y ( h z ) 30 S~ -(-> O) <u CO o > H 1—4 > I—I CO CO EH i—t o O > Figure 3. Typical v e l o c i t y s e n s i t i v i t y curve for Willmore Mk I I seismometer and AS-330 amplifier with -3 db points of f i l t e r set at 0.75, 12.5 Hz. Low output response at 24 db attenuation. Tape recorder response not included. 16 SHOT POINT LOCATIONS ROCKY MOUNTAIN TRENCH SEISMIC PROJECT 1972 - 1973 Shot Number Date Mine and P i t Charge s i z e ( l b s . ) L a t i t u d e (deg min) Longitude (deg min) O r i g i n (h Time m (U. T.) s) 1 June 8 K a i s e r not useable not o b t a i n e d 2 June 23 K a i s e r -Harmer 1 •^300 ,000 49*46.72' -114*49.83' 18 22 34.20 ± .14 3 June 23 F o r d i n g •V" 60,000 S0°12.32' -114*51.67' 22 18 00.74 i .02 4 June 24 K a i s e r -A d d i t 29 •^300,000 49°45.16' -114*48.50' 18 IS 23.07 ± .03 5 J u l y 7 K a i s e r -Harmer 11 •v-400,000 49 047.46' -114 o50.09' 20 00 58.86 ± .02 6 J u l y 8 K a i s e r -Harmer 1 -v-250,000 49*46.77' -114 049.85 , 18 16 06.11 t .02 7 J u l y 19 K a i s e r -A d d i t 29 •^366,000 49*45.57' -114°48.7S' 19 29 50.49 t .02 8 J u l y 21 K a i s e r -Harmer 11 ^600,000 49*47.39' -114*50.16' 19 29 34.68 f .03 9 Feb 22/73 K a i s e r -A d d i t 29 MOO,000 49°45.30' -114°48.65' 19 13 17.91 + .02 10 Feb 23 K a i s e r -Harmer 11 •v-500,000 49°47.32' -114*50.15' 20 59 06.07 + .02 11> 12* June June 20 20 F o r d i n g K a i s e r -Harmer 1 MOO,000 ^300,000 50*12.28' 49*46.75' -114 051.65' -114*49.84* 18 18 19 36 36 55 SO.69 50.96 20.52 + ± ± .05 .03 .03 13 June 21 K a i s e r - -v.300,000 49*47.46' -114*50.00' 18 14 29.88 • .02 Harmer 11 Notes: 1 F i r s t o r i g i n time r e p r e s e n t s i n i t i a l d e t o n a t i o n o f a s m a l l e r charge; second o r i g i n time r e p r e s e n t s d e t o n a t i o n of a l a r g e r charge i n a d i f f e r e n t p a r t of the p i t ; the d i s t a n c e between b l a s t s i s -v-1,000 f t . * No shot p o i n t r e c o r d i n g . L o c a t i o n i s g i v e n by the average o f a l l o t h e r Harmer 1 b l a s t s . Time i s based on P n a r r i v a l s at Mica a r r a y , the l a t t e r b e i n g c a l i b r a t e d by e a r l i e r b l a s t s . T y p i c a l b l a s t a r e a : (200 t o 300 f t by 600 t o 900 f t ) Shot p o i n t l o c a t i o n c a l c u l a t e d f o r c e n t e r o f b l a s t area s p e c i f i e d t o n e a r e s t 100 f t . The a s s o c i a t e d e r r o r s i n the c o o r d i n a t e s a r e : L a t i t u d e ± 0.02' Longitude ± 0.03' Table I. Shot point data. 17 f i r s t a r r i v a l seismic energy was very weak beyond about 400 km and was only recorded by v i r t u e of the excellent signal-to-noise r a t i o of the Mica array. This lack of ef f i c i e n c y i s probably due to the " r i p p l e - f i r e " technique i n which a pattern of shots i s l a i d out with up to several milliseconds delay between each detonation. However, this loss of compressional energy i s compensated by strong shear wave generation and indeed, large S-wave a r r i v a l s appear l a t e r on a l l the records, even for shots with very l i t t l e delay. This i s an important aspect of the p r o f i l e and w i l l be u t i l i z e d i n future studies to deduce a structure based on the integrated P- and S-wave data. Shots were numbered chronologically (Table I) and appended to the system name to designate records. (Thus, B6 i s the recording of shot 6 by system B; DA10 i s the recording of shot 10 at Mount Dainard.) Some locations (A2 and A3, B2 and B3, C2 and C3, B l l and B12) recorded blasts from both shot points (see the shaded triangles i n Figure 1), while the Mica array recorded signals from shots 9 to 13. The p r o f i l e i t s e l f extended from s i t e B2/B3 southeast of Radium (84 km from Fording, 112 km from Kaiser) to s i t e A13 near McBride (539 km from Kaiser). On the p r o f i l e just north of Radium a clustering of s i t e s can be seen. After the i n i t i a l survey was completed i n 1972, anomalous traveltime behaviour was observed i n this region, re s u l t i n g i n the addition of four s i t e s to this l o c a l i t y i n 1973. A l l f i e l d s i t e s i n the project lay near the east w a l l of the Trench, except for B4, C12 and B13 which lay on h i l l s i n the centre. Although attempts were made to locate s i t e s on bedrock wherever possible, the nature of the Trench made this task d i f f i c u l t . Consequently, many si t e s probably were underlain by several hundred feet of unconsolidated sediments. Shots from Fording turned out to be useful only with the A3 and B3 recordings south of Radium. Excessive timing noise i n system C obli t e r a t e d C3; B l l was very weak and emergent, and coda from an e a r l i e r b l a s t interfered with the a r r i v a l of shot 11 at the Mica stations. Table I indicates another problem with shot 11. Two shots were detonated, the larger one being delayed by about 0.3 sec from the f i r s t . The decision was made to use the e a r l i e r o r i g i n time, although i t s associated energy was less. Record B7 (north of DAI) was oblitera t e d by r i v e r noise and system malfunctions. Record C7 (beside CUM) recorded horizontal 18 components of ground motion only, while B6, B l l , B12, B13, C12, and C13 recorded only the v e r t i c a l and r a d i a l components. The Mica array, of course, recorded the v e r t i c a l component only. 19 PRELIMINARY DATA ANALYSIS D i g i t i z a t i o n After the data had been recorded on FM magnetic tape, d i g i t i z a t i o n was performed i n the Department of Geophysics and Astronomy Laboratory. The procedure can be described b r i e f l y as follows. The recorded signal i s played back on a Sanborn 3907B FM tape recorder and i s fed into a S c i e n t i f i c Data Systems A/D converter. Sample-and-hold amplifiers i n the converter allow simultaneous d i g i t i z a t i o n of four data channels. The converter i t s e l f receives instructions from an Interdata Model 4 computer which i s t i e d to a Teletype terminal. The d i g i t i z e d s i g n a l i s then dumped onto seven track magnetic tape. The d i g i t i z i n g i n t e r v a l i s controlled by a clocking system which triggers the A/D converter. To ensure a f a i t h f u l reproduction of the analog s i g n a l i n the time domain, a d i g i t i z i n g i n t e r v a l of 85 Hz (0.012 sec) was chosen. Therefore, since the Nyquist frequency of the d i g i t i z e d data was 42.5 Hz and the high cutoff of the AS-330 amplifiers was 12.5 Hz, any a l i a s i n g problems were completely eliminated. After d i g i t i z i n g , the data were demultiplexed and written on nine track tape i n blocks 11 - 12 seconds long for analysis on the U.B.C. IBM 360/67 computer. Minor tape speed variations between the systems were successfully accounted for i n the p l o t t i n g programs, res u l t i n g i n a maximum timing error of 0.005 sec at each point. The analog tape records were l a t e r spliced onto a single r e e l and stored as a backup. F i l t e r i n g Although many of the records showed excellent signal-to-noise r a t i o s , a substantial number required bandpass f i l t e r i n g . For these records power spectra of the background noise p r i o r to onset and of the f i r s t several seconds of P coda were calculated. A comparison between the two spectra for each record then gave an i n d i c a t i o n of the most ef f e c t i v e bandpass l i m i t s . A clear example of this method i s shown i n Figure 4a. The s i t e at A3 was plagued by large amplitude monochromatic 5 Hz noise which e f f e c t i v e l y disguised the seismic energy from a Fording shot (Figure 5a). However, the power spectrum of the a r r i v a l showed considerable energy i n the 6 - 8 Hz range, so that a bandpass f i l t e r from a 20 CM LU Q ID I — »-<o —'r\j Cu z: cn UJ > •—•a I—a . A3 NOISE 12.0 4.0 5.0 FREQUENCY (HZ) 8.0 10.D 12.D Figure 4a. Comparison of power spectra for p r e - a r r i v a l noise and for the seismic s i g n a l at A3. 8-C6 NOISE tn o X L D LU o 2 : cc LU > 1—1 I — a . _ J LU Cr: 1 r 4,0 6.0 FREQUENCY (HZJ ZA 8.0 C6 SIGNAL 10.0 12.0 -CCA 4.0 6.0 FREQUENCY (HZ) 8.0 10.0 12.0 Figure 4b. Comparison of power spectra for p r e - a r r i v a l noise and for the seismic s i g n a l at Cb. A3 UNFILTERED S 3 Figure 5 a . Seismic s i g n a l at A3 before and after application of Butterworth band-pass f i l t e r . C 6 UNFILTERED 1 S E C . FILTERED: 0 . 2 - 4 . 4 HZ , Figure 5b. Seismic signal at C6 before and after application of Butterworth band-pass f i l t e r . 24 5.7 - 11.8 Hz eliminated most of the unwanted noise (Figure 5a). A fourth order Butterworth bandpass f i l t e r was used throughout, and can be described by the following r e l a t i o n (Kanasewich, 1974): \Y(Q)\2 = ( 1 + fi2n)-l where u = — — x s a normalxzed frequency Y(£2) i s the transfer function (Oj,o)2 are the low and high frequency cutoffs n = 4 The data are convolved with the impulse response of the f i l t e r , then reversed and passed through the same f i l t e r again. This i s i n order to achieve zero phase s h i f t i n the f i l t e r e d s i g n a l . The re s u l t i n g f i l t e r response i s down by 6 db at the cutoff frequencies. Thus, care had to be taken i n the f i l t e r i n g process not to attenuate signal frequencies which were juxtaposed with the undesirable noise. Butterworth f i l t e r i n g was used also to eliminate much of the system C timing noise which occurred on records from shots 2 to 6. The power spectra of C6 background noise (Figure 4b) shows 3 Hz noise with a pronounced harmonic at 6 Hz and one at 12 Hz. The signal spectrum shows a predominance of energy i n the 1 - 4 Hz range, d i c t a t i n g the use of a 0.2 - 4.4 Hz f i l t e r as shown i n Figure 5b. The signal i s d e f i n i t e l y enhanced, but 3 Hz noise i s s t i l l present and cannot be removed by bandpass f i l t e r i n g . Noting that this timing noise appears to be i n phase across a l l three seismic channels, a simple technique was attempted i n which the trace of one seismometer, p r i o r to the onset of seismic energy, was physic a l l y subtracted from another. I t was hoped that this would at least delineate the f i r s t a r r i v a l , but the noise apparently i s not as regular across the three channels as i t appears to the eye. However, this technique may be worth additional e f f o r t . Formation of record sections In order to form record sections, calculations of traveltimes, distances and amplitude scale factors were necessary. Shot times from the shot point records were corrected for the blast-to-geophone distance and the difference between WWVB coded time and Universal Time (UT), 25 giving an o r i g i n time with an error of 20 -. 30 msec (Table I ) . Traveltimes were calculated using plots of the demultiplexed data blocks and WWVB/UT corrections were added. In addition, elevation corrections were made to bring a l l recording s i t e s to a 2000 feet above sea l e v e l datum, the approximate elevation of the Trench f l o o r . The r e s u l t i n g correction was quite small, the maximum change i n traveltimes being only 0.15 sec. Shot locations were calculated by the mining engineers from a mine gr i d t i e d to nearby bench marks. The assoc-iated errors are 0.02' l a t i t u d e and 0.03' longitude. Using a e r i a l photographs and 1:50,000 scale topographic maps, s i t e locations generally were determined to a s i m i l a r accuracy (approximately 40 - 50 m). Distances were calculated using a pre-existing program giving f i n a l errors i n the order of 0.1 km. Amplitude scale factors were deemed a very necessary factor as shown by Forsyth 0-973). D i g i t i z a t i o n factors were constant, but amplifier settings, s i g n a l frequencies, system responses, shot energies and distances were not. From the power spectra analysis, i t was observed that the seismic energy i n a l l records except those from shot 3 f e l l i n a narrow frequency band from 2 - 4 Hz. Records B3 and A3 showed energy i n the 6 - 8 Hz range (Figures 4a, 4b). Therefore, since the sig n a l frequencies are higher than the low cutoff frequency, i t can be assumed that the seismometers recorded ground v e l o c i t y (Figure 3). Nevertheless, this i s only an approximation near the low cutoff. Values of v e l o c i t y s e n s i t i v i t y were then picked from Figure 3 to coincide with the predominant signal frequencies recorded by the p a r t i c u l a r system. Once corrected f o r amplifier settings, this value became the i n i t i a l amplitude factor. I t i s an established fact (e.g. Forsyth, 1973.; Berry and Fuchs, 1973) that energy y i e l d from a blast bears l i t t l e r e l a t i o n to the charge s i z e . Accordingly, amplitudes of each shot were picked from short-period photographic records recorded at the standard stations PNT and 3ES, and an average of the two values was then used as the shot factor. I t was subsequently observed, for example, that although shot 4 was of s l i g h t l y less than average s i z e , i t produced by f a r the largest amplitudes. F i n a l l y , i t was decided to introduce a factor to account for geometrical spreading of the seismic energy along the p r o f i l e . 26 Since the weak P a r r i v a l s at the f a r end of the p r o f i l e are the most n severely affected by spreading, a factor i n v e r s e l y proportional to the head wave c o e f f i c i e n t was applied to the data. Cerveny and Ravindra (1971, p. 147) show that at large epicentral distances ( r ) , and other factors being constant, this c o e f f i c i e n t i s approximately proportional 2 2 to 1/r . Therefore, the data were m u l t i p l i e d by a factor of r to enhance the head wave a r r i v a l s , although this factor tends to over-emphasize body waves which attenuate with distance by a factor of 1/r. After normalizing with respect to the largest observable amplitude, the traces were consolidated i n the form of record sections. When these amplitude factors are combined, i t i s intended that the r e s u l t i n g energy pattern varies uniformly across the p r o f i l e and that amplitude anomalies i n the record section are s i g n i f i c a n t i n terms of underlying structure. 27 INTERPRETATION General features of the record section The normalized record sections, reduced by an average c r u s t a l v e l o c i t y of 6.5 km/s, are shown i n Figures 6 and 7. Figure 6 has been corrected for a l l amplitude factors except that f o r geometrical spreading, whereas amplitudes i n Figure 7 have been m u l t i p l i e d by the square of the distance. Even though the o v e r a l l amplitude correction factors at dif f e r e n t s i t e s •vary by over two orders of magnitude, a smooth v a r i a t i o n of energy with distance resulted. One exception to this pattern occurred i n the centre of the section where records A6, B6 and C6 had amplitudes seemingly too large by at least a factor of three. However, upon examining the f i r i n g pattern of shot 6, i t was discovered that the shots were detonated along the perimeter of a narrow, approximately e l l i p t i c a l shape, with long axis pointing north. Therefore, i t i s quite possible that the radiation pattern would show s i g n i f i c a n t differences between the east-west directions of SES and PNT and the southeast^northwest d i r e c t i o n of the p r o f i l e , and that the amplitudes of incident energy would not be the same. Since seismograms from a standard "U.S. st a t i o n to the southeast were unavailable, the amplitudes of these three records were reduced by a constant factor to f i t i n to the pattern. An obvious feature of the record section i s that no recordings were made within 80 km of the shot points. Due to l i m i t e d time and funds, i t was decided to forgo information about the surface layer. Thus, incident energy from the upper crust never appears i n the form of f i r s t a r r i v a l s . Records from 80 - 150 km show a branch of f i r s t a r r i v a l s with an apparent v e l o c i t y of 6.5 - 6.6 km/s and an intercept of 1.25 ± 0.15 sec. Although the f i r s t a r r i v a l at C13 was emergent, i t was picked with a high degree of confidence from the analog record. This branch of f i r s t a r r i v a l s has been interpreted as the head wave (P ) propagating along the surface of the c r y s t a l l i n e basement beneath the Rocky Mountains. I t should be noted that the higher frequency energy apparent i n A3 and B3 i s due to the source effect of the Fording explosion; traces A2 and A3 were recorded at the same lo c a t i o n , as were B2 and B3. With one exception, records from 80 - 150 km seemed to show no coherent branches of upper c r u s t a l energy. The exception i s the strong high frequency phase recorded on A3 at a REDUCED TRAVEL TIME T-D/6.5 (SEC) Q-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 83 6Z 30 reduced traveltime of 4.5 sec and on A2 at 5.75 sec, giving an apparent v e l o c i t y of approximately 5.0 km/s. The high frequency character of this phase at A2 has been attenuated by f i l t e r i n g i n order to improve the quality of the f i r s t a r r i v a l . I t has been interpreted as either an upper c r u s t a l phase or a l o c a l reverberation. Perhaps the most interesting phenomenon of the record section occurs from 150 to 200 km amidst a grouping of large amplitude phases. Beyond C13, f i r s t a r r i v a l s are extremely weak and are delayed from an extrapolated P branch by approximately 0.5 sec near 170 km and 1.0 sec near 190 km. On record B12 at 2.9 sec a very strong higher frequency (8 Hz) phase suddenly appears which i s not apparent on B13 at exactly the same distance. This phase dies out rapidly beyond B12 to B5, but seems to l i e on a branch with an apparent v e l o c i t y of 6.5 - 6.6 km/s, values i d e n t i c a l with those of the P phase. g Another s t r i k i n g feature i s the large amplitude secondary a r r i v a l s from about 160 km to 320 km near A7. These have been interpreted as r e f l e c t i o n s from the Moh.o at and beyond the c r i t i c a l angle. Synthetic seismograms to substantiate this interpretation appear l a t e r i n this section. Records from 279 to 444 km show a low amplitude f i r s t a r r i v a l of frequency 3 - 4 Hz, followed 0.5 seconds l a t e r by a stronger phase. The apparent v e l o c i t y of the f i r s t a r r i v a l phase i s 8.22 ± 0.04 km/s with an intercept of 10.3 ± 0.3 sec, and has been interpreted as the head wave t r a v e l l i n g along the Moho discontinuity (P ). As predicted by n the head wave c o e f f i c i e n t , i t s amplitude remains r e l a t i v e l y constant along the record section corrected by the distance squared factor (Figure 7). Except for an early onset by 0.2 seconds at CUM, t h i s phase l i e s on a remarkably straight l i n e and has been picked with a high degree of confidence. I t probably becomes a f i r s t a r r i v a l i n the v i c i n i t y of C5 (230 - 240 km), but timing noise on C5 and C6 and a passing t r a i n on A5 have e f f e c t i v e l y obliterated the evidence. Near 170 km the P^ branch appears to merge with the re f l e c t e d branch, but this i s rather vague. The larger phase 0.5 seconds l a t e r than P has been interpreted i n n terms of a low v e l o c i t y layer beneath the Moho. Over the l a s t 100 km of the p r o f i l e a most int e r e s t i n g late phase appears. I t i s f i r s t seen with confidence at TH0, approximately 9 - 1 0 seconds after P . The same phase i s prominent on A12 and A13; 31 a l l three appear to l i e on a l i n e ( s l i g h t l y concave upward) with apparent v e l o c i t y 5.6 ± 0.1 km/s and intercept -6.0 ± 1.7 sec. I t intersects the P branch at 277 ± 13 km and may be related to the n large amplitude, long duration 4 Hz phase on A8 at a reduced traveltime of 4 sec. Evidence for an interpretation of t h i s a r r i v a l as a converted phase w i l l follow. F i n a l l y , the l a s t feature of note i s the s i g n i f i c a n t change i n noise levels north of DAI. Immediately south of this region, records B6, A6 and A7 show excellent signal-to-noise r a t i o s , as do the Mica s i t e s TAB, CUM and DAI. A l l three f i e l d s i t e s were located on the va l l e y f l o o r and B6 was not even on bedrock. However, A8, B8 (also B7, not shown), A12 and A13 indicate a much higher background noise l e v e l , even though only A8 was not located on bedrock. Even THO records show a n o i s i e r character than do the other three Mica s i t e s . A very tentative suggestion i s that this increase i n noise l e v e l may be related to the occurrence of Proterozoic gneissic rocks which outcrop only i n a small zone around A8, B8 and at THO (Geological Map of Canada). A l l s i t e s south of DAI are located on lower Paleozoic sedimentary rocks. In addition, s i t e s A12 and A13 appear to be located along a long b e l t of mainly metamorphosed Hadrynian sedimentary rocks which occur nowhere else to the south i n the Trench. This may be an in d i c a t i o n either of poor coupling to basement rocks or of a shattered character i n the surface rocks. Interpretation technique Interpretation of the data i n terms of c r u s t a l structure r e l i e d heavily upon a computer routine (HRGLTZ) developed by R.A. Wiggins. A table of p-delta values i s i n i t i a l l y constructed from the data by evaluating the slopes of the traveltime. branches i n the record section. After this the program produces velocity-depth and T-delta curves based on Weichert-Herglotz integration and geometric ray theory. Provision exists i n the program for low v e l o c i t y zones but not for l a t e r a l inhomo-geneities such as f a u l t s . The program also calculates synthetic seismograms based on the quantized ray theory of Wiggins and Madrid (1974) , although this technique i s only approximate near large d i s c o n t i n u i t i e s i n v e l o c i t y . 32 Structure based on arrivals from 0 to 150 km Since the Trench is a detritus-filled feature and since many sites were not located on bedrock, a sediment thickness of about 200 meters with velocity 3.3 km/s (Lamb and Smith, 1962) was assumed. P-delta values corresponding to this velocity were included in the table and manipulated so that HRGLTZ produced the assumed thickness (Figure 8). However, inclusion of this layer has relatively l i t t l e effect on the rest of the profile, increasing the intercept times of later branches only slightly. The first problem encountered was to deduce a reasonable estimate of the upper crustal velocity. As mentioned earlier, records A2 and A3 show significant secondary energy with an apparent velocity of 5.0 km/s. Consequently, a model was attempted with a 5 km/s layer above the 6.5 km/s zone (dotted lines i f Figure 8). The upper T-delta plot in Figure 9 shows the best f i t of the two branches to the data, using a short period (T = 0.125 sec) Gram-Charlier wavelet with sharp onset to simulate the high frequency energy of the 5 km/s branch and the Fording source energy. As described by Jones and Morrison (1954), a Gram-Charlier wavelet is the negative sum of the fifth and sixth derivatives of the Gaussian error function. Figure 6 should be used for comparison since the HRGLTZ routine does not compensate for geometrical spreading. A relatively sharp gradient was introduced to increase the amplitudes of the reflected branch, in keeping with the large energy on the section. However, even i f the 5 km/s branch is extended further, the secondary synthetic amplitudes are not as large as desired. Doubt about the validity of this branch is increased when i t is observed that this phase only appears at the one recording location, and may just be a local reverberation. Finally, its associated velocity is lower than has been determined for the upper crust by other workers in the area. Chandra and Cumming Q.972) obtained a velocity of 6.16 km/s in the foothills of central Alberta, while Forsyth (1973) used a value of 5.6 km/s for an area about 200 km west of the Trench an 53°N. However, i t was felt that the velocities of 5.2 - 5.5 km/s obtained by Lamb and Smith (1962) in the Trench were more applicable. The higher velocity of 5.5 km/s (p = 0.18 s/km) was then chosen as a compromise and included in the p-delta table (solid line in Figure 8). Some form 33 DEPTH (KM) AO. 60. 80. DISTANCE (KM) 100. 120. 140. 160 Figure 8. Velocity-depth structure and p-Delta curves for two basement models. Dotted l i n e i s the 5.0 km/sec model. S o l i d l i n e i s the preferred 5.5 km/sec model. 34 DISTANCE (KM) DISTANCE (KM) Figure 9. Synthetic seismograms f o r the 5.0 km/sec model (top) and the 5.5 km/sec model (bottom). A Gram-Charlier wavelet with sharp onset i s used. The higher frequency wavelet corresponds to energy from Fording, whereas the lower frequency corresponds to energy recorded from Kaiser. 35 of smooth v e l o c i t y gradient i s necessary in. order to reduce the secondary amplitudes, since no coherent 5.5 km/s branch i s seen i n the data. Even so, the synthetic seismograms of the lower plot i n Figure 9 show secondary amplitudes s l i g h t l y larger than desired. This i s probably due to the lack of a constant unique v e l o c i t y i n the highly deformed upper crust. (The same form of wavelet as before was used with T = 0.35 sec to simulate the observed frequencies from Kaiser.) Q u a l i t a t i v e l y then, the velocity-depth structure shown by the s o l i d l i n e i n Figure 8 was chosen as the most reasonable f i t . This gives an average v e l o c i t y , including the gradient, of 5.7 km/s to a depth of 6.5 km. Underlying this layer i s the basement with v e l o c i t y 6.5 km/s. As can be seen from Figure 1, this i s the basement depth beneath the Western Ranges of the Rockies i n this region, and not the depth to basement beneath the Trench. Low v e l o c i t y zones The next problem was a suitable explanation for the time delay beyond 150 km. This phenomenon can be interpreted as the "shadow zone" eff e c t of a low v e l o c i t y layer, and a model with this structure has been achieved. C r e d i b i l i t y i s lent to this hypothesis by recent interpretations i n a s i m i l a r vein (e.g. Berry and Fuchs, 1973). A low v e l o c i t y zone beneath the Moho w i l l be described after the upper c r u s t a l zone. A time increment of 1.75 sec was chosen from the record section for the "shadow zone", this being the time delay between the traveltime branches at 150 km and 230 km (Figures 10, 11a). This produced an extremely thick (15.5 km) layer of v e l o c i t y 6.1 km/s at a depth of 9.1 km (Figure 10). To simulate the curvature and large amplitudes of the reflected branch from the Moho, a l i n e a r decrease i n p was modelled (from 300 km to 170 km i n Figure 10), producing a v e l o c i t y gradient from the low ve l o c i t y zone at a 24.6 km depth to the Moho at 56.2 km. A f i t of the traveltimes to the record section i s quite good (Figure 11a) . Since the HRGLTZ routine does not compensate for .. geometrical spreading, the corresponding uncorrected record section was used for comparison with the synthetic plot (Figure l i b ) . On the record section note especially how at distance 300 km the l a t e r "Pg" branch merges smoothly into the wide-angle r e f l e c t i o n s from the Moho, 36 a . " I _ j i i i i : i i i i c f l . 53. 100. J50. 200. 250. 300. 153. 400. 4S0. S00 DISTANCE (KM) Figure 10. Velocity-depth structure and p-Delta curve f o r the low v e l o c i t y zone inter p r e t a t i o n . 0 0 (!) 20 40 60 80 100 120 140 160 180 200 220 DISTANCE (KM) 320 340 360 380 400 420 440 460 480 500 520 540 Figure 11a. F i t of travel-times to the record section for the low v e l o c i t y zone interpretation. Figure l i b . Synthetic seismpgrams for the low ve l o c i t y zone inter p r e t a t i o n . A "Kaiser" Gram-Charlier wavelet i s used. 39 adding confidence to the interpretation of this "Pg" branch. Also note that the largest r e f l e c t e d amplitudes (on A4 and C4) occur approximately 30 to 40 km beyond the c r i t i c a l point. This r e s u l t agrees with the work of Cerveny (1966) who showed that, for a 3 - 4 Hz frequency and a r e f r a c t i v e index of 0.80 - 0.83 at the Moho, the maximum amplitude should appear about 40 km beyond the c r i t i c a l point. With two notable exceptions, the synthetic seismogram section of Figure l i b shows good agreement with the data. On the po s i t i v e side, the traveltime curvature and amplitudes of the Moho ref l e c t i o n s agree w e l l , as do the amplitudes of the a r r i v a l s . However, the f i r s t exception to the f i t i s that the amplitudes of the P^ branch between 30 and 140 km do not attenuate as rap i d l y beyond 140 km as i s indicated by the data. As expected, i t also does not f i t the observed time delay of the f a i n t f i r s t a r r i v a l s behind an extrapolated P branch i n this g region. This may be due to the inherent inaccuracy of the HRGLTZ routine or i t may be evidence for l a t e r a l inhomogeneities. A s i m i l a r conclusion can be reached for the second exception: although the l a t e r "P " branch on the synthetic record section does show a large amplitude at 170 km, the amplitudes at some distance to either side are also quite large. Although the data show a large amplitude near 170 km (B12), there i s no evidence for energy corresponding to t h i s phase a r r i v i n g before 170 km, and "P " amplitudes beyond 170 km are strongly attenuated. Noting the fact that no other studies have found such a thick low v e l o c i t y zone i n the upper crust, the fact that i t may be p e t r o l o g i c a l l y impossible, and the imperfect f i t of the synthetic seismograms, the int e r p r e t a t i o n of a sub-basement low v e l o c i t y zone was rejected i n favour of a model involving l a t e r a l s t r u c t u r a l changes. However, the interpretation of a sub-Moho low v e l o c i t y zone seems more plausible. After a suggestion derived from Barr (1971), the strong energy 0.5 seconds after P^ was modelled as an ef f e c t of a low v e l o c i t y zone immediately underlying the Moho. This e s s e n t i a l l y means that body waves refracted j u s t beneath the Moho are delayed s l i g h t l y by the low 'velocity zone, and exhibit p r a c t i c a l l y the same apparent v e l o c i t y as the P^ phase. A layer 7.2 km thick, with v e l o c i t y 7.9 km/s, at a depth of 63.0 km gives the best f i t . The synthetic 1 record section i n Figure l i b shows that this secondary phase i s p a r a l l e l to the P^ branch and has s i g n i f i c a n t l y larger amplitudes beyond 280 km -features which are evident i n the record section of Figure 11a. 40 Fault models A downthrown f a u l t w i l l introduce a discontinuous delay i n the traveltime curve. A technique developed by the w r i t e r , details of which are given i n the Appendix, enabled the use of the HRGLTZ routine to model th i s type of discontinuity. The purpose of the technique i s to produce a down-fault velocity-depth structure. However, the HRGLTZ routine calculates h o r i z o n t a l l y layered models only, so that there must be a time difference between the actual traveltime branches i n the data and the traveltime branches calculated by HRGLTZ for a down-fault structure. Making certain assumptions about the ray paths beyond a f a u l t , this time difference can be calculated. The HRGLTZ routine then gives the down-fault depth, from which the throw of the f a u l t can be calculated. However, a word of caution i s advisable since several assumptions are made. One i s that the shot point-to-fault distance must be much greater than the f a u l t throw. As borne out by the r e s u l t s , t h i s assumption i s v a l i d . Secondly, average v e l o c i t i e s for the two cru s t a l layers are used, introducing a certain amount of inaccuracy i n calculating c r i t i c a l angles. Thirdly, the basic traveltime equations of geometric ray theory are used, even though, a detailed understanding of ray behaviour at a l a t e r a l discontinuity has not been developed adequately. Since the HRGLTZ routine does not handle l a t e r a l inhomogeneities, synthetic seismograms are not relevant. Only the traveltimes and a qu a l i t a t i v e discussion of amplitudes are meaningful. Such a discussion of amplitudes i s perhaps appropriate before a description of the traveltime plots. From 150 km to 220 km the f i r s t a r r i v a l s are very weak and appear to d r i f t upwards u n t i l they are no longer apparent past A4 CFigure 7). This weak phase could be interpreted as a d i f f r a c t i o n from the upper edge of the f a u l t . The strong anomalous amplitude on B12 can tentatively be explained as a focussing effect of rays incident upon the concave lower edge of the f a u l t (Barr, 1971). This phenomenon would tend to be very l o c a l i z e d and would not appear on adjacent records. There i s no such large amplitude at B13, although the distance from the shot point i s the same as B12. This could indicate a difference i n shape of the f a u l t surface beneath these s i t e s , since they were located several kilometers apart within the Trench. 41 Three c r u s t a l models w i l l be described. The f i r s t assumes a basement f a u l t which does not extend to the Moho; the second assumes a uniform f a u l t throughout the cru s t a l section. The t h i r d model calculates the f a u l t throw at the Moho based on an assumed up-fault r e f l e c t i o n from the Moho. Section 1 of the Appendix outlines the method by which the f a u l t throw on the basement was calculated. Assuming that the 6.5 km/s branch beyond 170 km has been delayed by the f a u l t , then a HRGLTZ branch which i s 0.89 sec sooner w i l l give the down-fault structure. The down-fault depth to basement i s 12.1 km, giving a f a u l t throw of 5.6 km. Section 2.1 explains the necessary corrections which must be applied to the intercept i f i t i s assumed that the f a u l t does not extend to the Moho. Using a s i m i l a r argument to the above, the intercept of the HRGLTZ model must be delayed from the P n intercept i n the data By 0.54 sec. The dotted l i n e i n Figure 12a represents the up-fault structure, showing that the 6.5 km/s layer extends with a v e l o c i t y gradient from 6,5 km depth, to 61.2 km at the Moho. The down^-fault structure Csolid l i n e i n a l l diagrams) has a s i m i l a r c r u s t a l thickness, and shows the f a u l t throw of 5.6 km at the basement. As explained e a r l i e r , the v e l o c i t y gradient i s necessary for a correct f i t of amplitudes and curvature of the Moho r e f l e c t i o n s . Figure 12b shows the corresponding traveltimes superimposed, for c l a r i t y , upon the record section corrected for geometrical apreading. Except for the up-fault P intercept, a l l other model branches are p a r a l l e l and delayed from the data traveltimes by the amounts calculated. Section 2.2 of the Appendix details the calculations made assuming the f a u l t , as calculated previously, extends throughout the cru s t a l section with a constant throw of 5.6 km. For th i s case, the P^ branch i n the data i s delayed by 0.01 sec from the HRGLTZ P n branch. Figures 13a and 13b show the up-fault and down-fault structures and the traveltimes which r e s u l t . Up-fault the 6.5 km/s layer extends from a depth of 6.5 km to 52.2 km at the Moho. The corresponding layer i n the down-fault structure extends from 12.1 km to 57.8 km. Again, a gradient must be included above the Moho. The t h i r d model was an improvement on the second i n that i t was an attempt to determine independently the throw of an assumed f a u l t 42 40. DEPTH (KM) 200. 250. 2i!0, DISTANCE (KM) 500 Figure 12a. Velocity-depth structure and p-Delta curves for a model with a basement f a u l t and an unfaulted Moho. Dotted l i n e s represent the up-fault model. R13 6' '20 '40 '60 '80 ' 100" 120' 140' 160' 180 200 220 DISTANCE (KM) 320 340 360 380 400 420 440 460 480 500 520 540 Figure 12b. Traveltime curves for the velocity-depth structure i n Figure 12a. Dotted and s o l i d lines indicate traveltimes for models based on the up-fault and down-fault structures, respectively, Model traveltimes are offset from the actual data to account for the perturbed ray paths due to the f a u l t . These models assume that the basement f a u l t does not extend to the Moho. 44 100. 150. 200 . 250 . 300. DISTANCE (KM) 350. 400. 450. 500. Figure 13a. Velocity-depth structure and p-Delta curves for a model with constant f a u l t throw throughout the c r u s t a l section. Dotted lines represent the up-fault model. u 20" 40 60 80 100 120 140 160 180 200 220 DISTANCE (KM) 320 340 360 380 400 420 440 460 480 500 520 540 Figure 13b. Traveltime curves for the velocity-depth structure i n Figure 13a. These models assume a f a u l t of constant throw throughout the c r u s t a l section. 46 40. DEPTH (KM) 50. 200 . 250 . 300. DISTANCE (KM) Figure 14a. Velocity-depth structure and p-Delta curves for a model which f i t s an assumed up-fault P intercept, n Dotted lines represent the up-fault model. A13 20 40 60 80 100 120 140 160 180 200 220 DISTANCE (KM) 320 340 360 380 400 420 440 460 480 500 520 540 Figure 14b. Traveltime curves for the velocity-depth structure i n Figure 14a. These models assume that the secondary a r r i v a l on C13 and B4 i s an up-fault r e f l e c t i o n from the Moho. 48 on the Moho. I t was based on the assumption that the secondary energy of C13 and B4 represented n e a r - c r i t i c a l angle r e f l e c t i o n s from the up-fault Moho. From the previous two models the c r i t i c a l point can be expected i n the range 147 - 174 km; C13 and B4 are located at 151 km and 162 km, respectively. The amplitudes are not overly large i n r e l a t i o n to the down-fault r e f l e c t e d branch, as expected for records at the c r i t i c a l point (Cerveny, 1966). The delay time between this early "branch" and the l a t e r P branch was picked from the record section n r as 1.5 sec. Following the method of Section 2.3, the down-fault Pn branch i n the data was delayed by 0.04 sec from the HRGLTZ P n branch. This gave a down-fault cr u s t a l thickness of 57.6 km (Figures 14a and 14b). A HRGLTZ model was then run for the up-fault structure, with a minor delay from the up-fault P^ "branch" to account for the di f f e r e n t thickness of the down-fault upper c r u s t a l layer. This gave an up-fault c r u s t a l thickness of 50.8 1cm, and a resulting f a u l t throw on the Moho of 6.8 km. Late a r r i v a l s beyond 400 km A further challenge of the section was to interpret the strong phases appearing at reduced traveltimes of 4 to 8 sec on A8, THO, A12 and A13. To this end, p a r t i c l e motion diagrams were made from the analog records. The phase on A12 and A13 showed marked shear wave motion i n the v e r t i c a l - r a d i a l plane (Figure 15), but the A8 phase had a rather confused motion. I t i s most l i k e l y a P-wave, although the large amplitude occurring about s i x seconds after phase onset exhibits S-wave ch a r a c t e r i s t i c s . The r e l a t i o n between this phase and the S-waves on A12 and A13 has not been resolved. However, lacking any horizontal information from seismograms recorded from THO, these l a t e r phases were assumed to be S on the basis of their s i m i l a r character to the A12/A13 a r r i v a l s . Therefore, i t was assumed that this phase had undergone some form of conversion from P to S within or at the base of the crust. Although the idea of a converted multiple has been proposed, only an interpretation as an S^ phase has been attempted. Unfortunately, the P^ phase cannot be picked with any confidence beyond THO, so that the following conclusions are mainly conjectural. However, the weakness of the P phase i s not s u r p r i s i n g , especially i f most of the compressional 49 V E R T I C A L 3-r -3-1-TR A N S V E R S E 3-R A D I A L Figure 15. P a r t i c l e motion diagram of the A12 large amplitude phase a r r i v i n g at a reduced time of 7 seconds. Time span = 1.3 seconds. 1 axis d i v i s i o n = 38 millimicrons of ground displacement. 50 energy has been converted to shear energy. If i t i s assumed that this phase f i r s t appears at THO, then i t s delay time from the P^ phase at this point should give an approximation to the depth of conversion. Assuming the crus t a l v e l o c i t i e s of the f a u l t models and a Poisson's r a t i o of 0.25, a calculation gives approximately 52 km for this depth. This i s 6 km less than the cru s t a l thicknesses for models with a faulted Moho. However, i t i s of the same order of magnitude as the Moho depth calculated for the southern part of the p r o f i l e . Allowing for the angle of incidence, this conversion would take place i n the v i c i n i t y of the Moho beneath DAI. The rather high apparent v e l o c i t y of 5.6 km/s can be explained by a southeasterly dip on the Moho beyond DAI. Again using average cr u s t a l v e l o c i t i e s and a Poisson's r a t i o of 0.25, the required dip would be 11°. Considering the assumptions made, this dip calculation can only be approximate. In addition, i t cannot be substantiated by the present seismic data. 51 DISCUSSION OF THE RESULTS The basement near 50°30'N, 115°W From the Rocky Mountain Trench seismic data, a depth of 6.5 km to a layer with P-wave v e l o c i t y 6.5 km/s has been calculated along a northwesterly-striking l i n e beneath the Western Ranges of the Rockies. From seismic r e f l e c t i o n studies, Bally et a l (1966) have shown that the basement under the western Rocky Mountains near 49°15'N dips gently to the southwest. And from a geologic reconstruction, Price and Mountjoy O-970) reached reached a s i m i l a r conclusion for the basement near 51°30'N. However, since the refraction p r o f i l e i s on s t r i k e with the regional trend of the C o r d i l l e r a , s i g n i f i c a n t dip along th i s l i n e i s not expected, and therefore, the apparent v e l o c i t y of 6.5 km/s i s probably close to the true v e l o c i t y . Data from both shot points seem to agree, so that these results may be taken as average values for the region. As expected for a feature related to the Precambrian Shield, the basement refractor -velocity of 6.5 km/s agrees better with data to the east than to the west. Chandra and Cumming (1972) have mapped a 6.5 km/s surface near 50°30'N i n Alberta, whereas at equivalent depths i n the Intermontane B e l t , White et a l (1968) and Forsyth (1973) found v e l o c i t i e s of only 6.1 and 6.2 km/s, respectively. A survey by Hales and Nation 0-973) ju s t south and west of the Trench found a v e l o c i t y of 6.41 km/s at 22 km depth beneath a 6.0 km/s layer. The seismic r e f l e c t i o n results of Bally et a l (1966) indicate a basement depth i n their southern p r o f i l e of about 7.9 km (26,000 f t ) below a datum at 2000 feet above sea l e v e l . According to Kanasewich et a l (1969) and Dragert (1973), a major Precambrian r i f t zone i n the basement st r i k e s northeast between this region and the area of our p r o f i l e . Therefore, i t i s not surprising that the agreement between the depth of Bally et a l (1966) and that calculated i n this study i s not more exact. I t must also be remembered that the depth of 6.5 km i s dependent upon the choice of an upper c r u s t a l v e l o c i t y . Including a v e l o c i t y gradient, the average v e l o c i t y i s 5.7 km/s. I f this v e l o c i t y i s too high, then the basement depth may be overestimated by as much as one kilometer. Unfortunately, the secondary a r r i v a l data show no 52 clear evidence for a uniform v e l o c i t y with a coherent branch. In such a complexly thrusted region, t h i s i s not unexpected. Low v e l o c i t y zones The upper c r u s t a l structure determined with a low ve l o c i t y zone i s not strongly supported by the w r i t e r , since the synthetic amplitudes f i t i s not good and the required layer thickness seems unreasonably large from a pet r o l o g i c a l viewpoint. However, several relevant points do merit discussion. Mueller and Landisman (1966) synthesized evidence for a low ve l o c i t y zone i n the upper crust. Quoting r e s u l t s form previous workers, they concluded that seismic v e l o c i t i e s i n granites go through a maximum at depths of around 5 - 10 km below the surface. Thus, a low v e l o c i t y zone beginning at 9.1 km i s not unreasonable. In an extensive interpretation of seismic data around the Grenville Front, Berry and Fuchs (1973) have included a low ve l o c i t y channel at a depth ranging from 5 - 12 km with a -maximum thickness of 3 - 5 km beneath the Front i t s e l f . Their conclusion i s that this thickening may be an ind i c a t i o n that the Front i s a zone of weakness. I f this i s so, then perhaps a s i m i l a r thickening under the Trench i s also an in d i c a t i o n of weakness. C i t i n g evidence for the effect of pore pressure on P-wave v e l o c i t i e s , they also conclude that water hydration i n the upper crust could produce a low v e l o c i t y zone. This s i t u a t i o n would arise i f the pore pressure were to increase suddenly i n r e l a t i o n to the confining pressure. Although the sub-basement low v e l o c i t y zone appears implausible, the 7.9 k/s layer beneath the Moho f i t s the data w e l l . Q u a l i t a t i v e l y , i t s thickness of about 7 km i s i n better agreement with findings that such p e t r o l o g i c a l changes take place over small distances. However, a petrologic explanation of this low v e l o c i t y zone w i l l not be attempted. Other workers (e.g. Forsyth, 1973) have s a t i s f a c t o r i l y modelled this phenomenon by including a v e l o c i t y gradient at depth beneath the Moho. However, the resulting t r i p l i c a t i o n does not properly describe the seeming p a r a l l e l i s m of the two branches i n the Trench data. I t i s also i n t e r e s t i n g to note that exactly the same a r r i v a l pattern was discovered by Cumming and Kanasewich (1966) and was interpreted as a conversion at shallow depth i n the Cretaceous section of Alberta. 53 These three interpretations have been advanced to explain what i s probably the same phenomenon. Somehow, a choice w i l l have to be made among them. The basement f a u l t The apparent time delay beyond 150 km can reasonably be explained by a normal f a u l t s t r i k i n g across the p r o f i l e with a downthrow of 5.6 km to the northwest. The good alignment of the delayed P branch with wide angle r e f l e c t i o n s from the Moho i s taken as evidence that the down-fault depth remains r e l a t i v e l y constant at least as far north as TAB. The calculated down-fault depth i s 12.1 km, but the error could be as much as 1 km due to the inherent error of the calculations and the choice of an upper c r u s t a l v e l o c i t y . S l i g h t l y north and east of this proposed f a u l t , Chandra and Cumming (1972) have mapped the surface of the 6.5 km/s layer at approximately 12 km. They have also extrapolated this surface beneath the Trench at a depth of about 20 km. However, the evidence for this i s not conclusive i n their data. Price and Mountjoy (1970) calculated the depth to basement as 11 km beneath the Trench north of Golden. This was estimated by extrapolating from a depth calculation at the B.C. - Alberta border (from formation thicknesses i n the thrust sheets) and a d r i l l hole to the east. I f i t i s assumed that the l a s t up-fault P a r r i v a l appears between 145 - 155 km, then the f a u l t must be located at a distance of 133 - 143 km from the Kaiser shot point. On the map this zone occurs s l i g h t l y south and to the east of Radium, extending several kilometers to the northwest. Although this f a u l t has no obvious surface geologic expression near Radium, this may only r e f l e c t i t s Precambrian nature. Later tectonic a c t i v i t y would have been superimposed over a pre-existing f a u l t ; the decollement zone of thrusting lay along the basement surface and did not involve basement rocks (Bally et a l , 1966). From gravity and magnetic data, the s t r i k e of this f a u l t appears to be northeasterly. In t h i s region contours on the Bouguer gravity anomaly map show a pronounced change i n s t r i k e to the northeast from the regional northwest trend; anomaly values decrease to the northwest, implying a decrease i n density (Figure 16). Patterns on the residual t o t a l magnetic f i e l d map (Figure 17) also show northeasterly trends i n 54 Figure 16. Eouguer gravity anomaly map of the Southern Rocky Mountains. The outline of the Trench i s indicated by the dashed l i n e s . Contour i n t e r v a l : 10 m i l l i g a l s Scale: 1:5,000,000 (produced by the Observatories Branch, Department of Energy, Mines & Resources, 1969) Figure 17. Residual t o t a l magnetic f i e l d map for southern Alberta and southeastern B r i t i s h Columbia (Kanasewich et a l . , 1969). Outline of the Trench i s indicated by the dashed l i n e s . . 56 the v i c i n i t y of 51°N, 116°W. A decrease from +200 gammas to -100 gammas seems to correlate with the zone of time delay seen i n the seismic data. This same diagram shows the marked r i f t feature of Kanasewich et a l (1969) to the south, a northeasterly trending zone characterized by a low of -300 gammas. In further support of t h i s s t r i k e d i r e c t i o n , northeasterly trending lineaments have been widely recognized i n the Precambrian Shield of western Canada (Burwash, 1965). Possible significance of the f a u l t with respect to Trench o r i g i n There i s one s i g n i f i c a n t alternative interpretation involving a basement f a u l t . Figure 1 shows that the ray path from Kaiser intersects the Trench i n precisely the same area over which the time delay i s observed. Could this mean that the basement underlying the Rockies actually "disappears" beneath the Trench? I f the Trench i s an ancient zone of weakness, could not the basement and overlying rocks have been so deformed as to disguise an o r i g i n a l smooth surface? Of a l l the theories of Trench o r i g i n , the idea of a zone of weakness has held the most appeal. Daly (1912) f i r s t wondered i f there was a genetic connection between the Trench and an ancient shore-line. Walker (1926) and Evans (1933) l i k e d the idea, postulating that "a zone of weakness would exi s t i n the basement rocks that might be r e f l e c t e d during the compression of l a t e r times." Both Henderson (1959) and Thompson (1962) believed that the zone of f a u l t i n g along the Trench was related to the margin of an ancient continent, and that i t marked the eastern hinge-line of the Cordilleran geosyncline. Leech (1964, 1965) argued for a fundamental basement control hypothesis - perhaps Mesozoic transcurrent f a u l t s , related to the cratonic boundary, have reasserted themselves through the fractured upper crust transported to the east above them. Although there i s no evidence for Cenozoic s t r i k e - s l i p movement along the Trench, Roddick (1967) and Tempelman-Kluit (1972) have described extensive Cretaceous transcurrent f a u l t i n g along the T i n t i n a Trench to the north (Figure 1). Berry et a l (1971) suggest that movement along these two f a u l t s may have been related; Souther (1970), Monger et a l (1972) and Gabrielse (1972) f i n d considerable evidence for transcurrent movement throughout the C o r d i l l e r a before the mid-Jurassic period. Geophysical evidence that the Trench actually does mark the western edge of the craton was stated e a r l i e r and has been synthesized 57 by Berry et a l (1971). They also make the point that the present thermal disturbance may have reactivated this old zone of weakness between cru s t a l blocks. And f i n a l l y , i n a study of world-wide "geomagnetic v a r i a t i o n anomalies", among which the Trench i s included, Law and Riddihough (1971) have observed that a l l of them are related to plate margins. A l l of this supports the idea that the basement rocks of the Trench were reworked over a long period of time, eliminating any sharp di s c o n t i n u i t i e s which may have been present o r i g i n a l l y . The lack of a r e f l e c t i o n from the Trench basement i n the data of Bally et a l (1966) could also be taken as supporting evidence for this theory. Even the most recent orogenic episode i n the Rocky Mountains was characterized by alternate compression (thrusting) and tension (normal faulting) (e.g. Mudge, 1970). I f future geophysical studies indicate that evidence for a normal f a u l t s t r i k i n g across the Trench i s meagre, then this can only mean strong support for this theory of Trench o r i g i n . Crustal thickness The v a l i d i t y of the c r u s t a l models naturally depends to a great extent on the lower c r u s t a l v e l o c i t y and the presence of dip on the Moho, which cannot be detected by an unreversed p r o f i l e . The selection of 6.5 km/s as the P v e l o c i t y i s unambiguous. I t certainly cannot be any l e s s , although i t could be as high as 6.6 km/s. The constraint provided by the branch of ref l e c t i o n s from the Moho i s also unambiguous. I t forces the inclus i o n of the v e l o c i t y gradient seen i n Figures 10, 12a, 13a, and 14a, re s u l t i n g i n an average lower c r u s t a l v e l o c i t y of 6.8 km/s. Although no evidence for a deeper cr u s t a l layer was found i n this data, Chandra and Cumming (1972) included a 7.15 km/s layer at a depth of 31 km under the f o o t h i l l s . Since the cru s t a l structure beneath the Trench appears to be related to that found further to the east, the seemingly high average v e l o c i t y of 6.8 km/s i s quite reasonable. The question of dip on the Moho (and the basement) cannot be resolved, but the Bouguer gravity anomaly contours i n Figure 16 suggest r e l a t i v e flatness along the p r o f i l e . Also, the P v e l o c i t y of 8.20 km/s obtained beneath the Trench by n Chandra and Cumming (1972) agrees w e l l with the observed value of 8.22 ± 0.04 km/s observed i n this study. 58 Since the apparent v e l o c i t i e s i n the p r o f i l e appear to be close to the true v e l o c i t i e s , the existence of a thick crust i s strongly suggested. Previous extrapolations have a l l indicated a depth of less than 50 km (Chandra and Cumming, 1972; D.A. Forsyth, o r a l communication, 1973). But even the low v e l o c i t y zone model gives a depth to the Moho of 56 km. I f a deep cru s t a l layer of higher v e l o c i t y were to be included, this value would be even greater. The models calculated for a basement f a u l t give a c r u s t a l thickness to the northwest of 61 km or 58 km, depending on whether or not the Moho i s faulted. Uprfault, the corresponding depths are 61 km and 51 - 52 km. Owing to the inaccuracies of the c a l c u l a t i o n s , these depths may be i n error by as much as 2 km. Gravity models by Stacey (1973) for a p r o f i l e across the C o r d i l l e r a from 49°N to 51°N have a c r u s t a l thickness of 60 km beneath the Rocky Mountains. Therefore, as explained e a r l i e r , the density change which he assumed to occur beneath the Trench does not have to be moved to the east. This also agrees with the discovery that the upper cru s t a l v e l o c i t y beneath the Trench i s s i m i l a r to that found i n the east. Price and Mountjoy (1970) claim that the t o t a l thickness of the crust has been increased by at least 8 km due to the accretion at the top which occurred i n conjunction with the stacking of thrust sheets. The Grenville Front, a s i m i l a r long boundary between two di f f e r e n t geologic provinces, also exhibits a r e l a t i v e thickening of the crust (Berry and Fuchs, 1973). In a geomagnetic depth-sounding p r o f i l e across the Trench, Dragert (1973) also required considerable c r u s t a l thickening. F i n a l l y , the Bouguer gravity anomaly along the Trench between 51°N and 53°N i s very low, i n places less than -210 mgal. Although quantitative modelling has not been performed, this gravity mimimum provides additional evidence for a thick crust. A f i n a l judgement regarding the Moho topography i s not r e a l l y possible with this data alone. The structure shown i n Figure 14a i s preferred since i t i s best supported by evidence i n the data. Since Kanasewich et a l (1969) have obtained strong evidence for a f a u l t throughout the c r u s t a l section, i t may be reasonable to assume a s i m i l a r phenomenon i n a s i m i l a r feature to the north. Consequently, the more extreme c r u s t a l thickness of 61 km could be rejected i n favour of the more conservative structure of 51 km south of Radium and 58 km,to the north. 59 However, i f i t i s discovered that the large amplitudes on C13 and B4 are not related to the Moho, this interpretation w i l l have to be revised-Converted S phase The results of this section should be viewed with a certain amount of skepticism. What i s d e f i n i t e l y known about this l a t e branch at the north end of the p r o f i l e i s that i t i s a shear wave and that i t travels with a high apparent v e l o c i t y of 5.6 km/s. Its i n t e r -pretation as an S^ converted phase i s only tentative - i t i s assumed to convert from P to S after s t r i k i n g some form of discontinuity along the Moho, subsequently t r a v e l l i n g along this interface as an S wave. n Also, an upwards dip on the Moho beyond DAI cannot be confirmed by the weak P^ a r r i v a l s beyond THO. However, there are several interesting phenomena which may relate to this phase. One i s that the gravity values at the north end of the p r o f i l e Increase rapidly beyond about THO, whereas they are very f l a t over most of the southern part of the p r o f i l e . This could be evidence for a rapid thinning of the crust i n the northwest. I t could also be evidence for a c r u s t a l f a u l t with downthrow to the south, perhaps ind i c a t i n g that the Rocky Mountains between 51°N and 53°N have depressed the crust-mantle boundary. The previously mentioned results of Caner et a l (1971) show a change i n d i r e c t i o n of the geomagnetic t r a n s i t i o n zone anomaly somewhere between THO and TAB. This change would occur within the lower crust and upper mantle and may be related to the S-wave conversion at this depth. F i n a l l y , the anomalous outcroppings of Proterozoic rocks between DAI and THO may be i n d i r e c t l y related to the question. A suggestion has been made (P. Simony, o r a l communication, 1972) that these rocks have actually been derived from the c r y s t a l l i n e basement. I f this i s so, i t would appear that the basement i n this region has been much more active than i n other parts of the Trench. Thus, i t could be an expression of a more widespread disturbance at depth i n the crust, a condition which could cause the observed conversion. 60 CONCLUSION A seismic refr a c t i o n survey has been conducted i n the southern Rocky Mountain Trench. Detailed attention to amplitude scale factors has produced a record section which shows a smooth v a r i a t i o n of energy with distance. An analysis of this section has led to the following conclusions: (1) A basement anomaly has been detected j u s t south of 51°N, 116°W near Radium. I t has been interpreted as a normal f a u l t with a downthrow of 5.6 ± 1 km to the northwest. Comparison with l o c a l gravity and magnetic trends and with known lineaments i n the Precambrian Shield has led to the conclusion that t h i s f a u l t s t r i k e s northeasterly across the Trench. C2) I f more detailed studies do not confirm the existence of a normal f a u l t s t r i k i n g across the Trench, the seismic data w i l l then strongly support the theory that the Trench macks an old zone of weakness i n the basement. By this i n t e r p r e t a t i o n , the basement could be down-faulted and/or strongly reworked west of the east w a l l of the Trench. (3) A preferred interpretation has the f a u l t extend throughout the entire c r u s t a l section with up- and down-fault structures as shown i n Figure 14a. Up-fault, a 5.5 km/s layer extends with a gradient to a depth of 6.5 ± 1 km, below which l i e s a 6.5 km/s layer to the Moho at 51 ± 2 km. Down-fault, the corresponding depths are 12.1 ± 1 km and 58 ± 2 km. A v e l o c i t y gradient above the Moho i s necessary. The e x i s t -ence of such a thick crust i s strongly suggested by other geophysical and geological studies. (4) An interpretation based on a thick sub-basement low v e l o c i t y zone was achieved but thought to be u n r e a l i s t i c . However, even this model gives a thick (55 ± 2 km) c r u s t a l section. (5) An analysis of a r r i v a l s delayed from the P^ phase i s consistent with an interpretation of a low v e l o c i t y zone 7 km t h i c k , 8 km beneath the Moho. Other explanations have been offered for these a r r i v a l s , and a choice w i l l have to be made among them. (6) Study of a converted S phase has led to the tentative conclusion that the Moho surface dips steeply to the southeast beyond 53°N. Although t h i s conclusion i s based on rather slim seismic evidence, i t i s supported by int e r e s t i n g g r a v i t y , magnetic and geologic 61 phenomena i n the region. This study has interpreted several features of the Rocky Mountain Trench seismic data, and pointed the way to l a t e r studies. A detailed gravity/magnetic/seismic survey along the Trench from 49°N to 51°N would obtain more data relevant to the basement anomaly discovered i n this data. I t would also serve to t i e i n the r i f t feature of Kanasewich et a l (1969). Some gravity and magnetic data i s already available. The velocity-depth structure as determined i n this thesis should be refined by a gravity interpretation along the entire p r o f i l e . This may also help to c l a r i f y the theories of a faulted Moho at 51°N and a steep, southeasterly dipping Moho beyond 53°N. F i n a l l y , there i s much more to be gleaned from the seismic data. A more detailed study of the motions i n the P-wave data must be integrated with an interpretation of the extensive S-wave a r r i v a l s . When this i s accomplished, a more complete understanding of this enigmatic v a l l e y w i l l be obtained. 62 REFERENCES B a l l y , A.W., P.L. Gordy, and G.A. Stewart. Structure, seismic data and orogenic evolution of Southern Canadian Rocky Mountains. Bull. Can. Soo. Petrol.. Geol., 14, 337-381, 1966. Barr, K.G. Crustal r e f r a c t i o n experiment: Yellowknife 1966. J. Geophys. Res., 76, 1929-1947, 1971. Berry, M.J., W.R. Jacoby, E.R. N i b l e t t , and R.A. Stacey. A review of geophysical studies i n the Canadian C o r d i l l e r a . 'Can. J.. Earth Sci-., 8, 788-801, 1971. Berry, M.J., and K. Fuchs. Crustal structure of the Superior and Grenville Provinces of the Northeastern Canadian Shield. Bull. Seism. Soo. Am., 13, 1393-1432, 1973. Burwash, R.A. Basement Architecture of Western Canada. Alta. Soo. Petrol. Geol. 15th Field Conference Guide Book, 280-288, 1965. Caner, B., D.R. Auld, H. Dragert, and P.A. Camfield. Geomagnetic depth-sounding and c r u s t a l structure i n western Canada. J. Geophys. Res., 76, 7181-7201, 1971. Cerveny, V. On dynamic properties of refl e c t e d and head waves i n the n-layered earth's crust. Geop. J. Roy. Astr. Soc, 11, 139-147, 1966. Cerveny, V., and R. Ravindra. Theory of Seismic Head Waves, 312 pp. University of Toronto Press, 1971. Chandra, N.N., and G.L. Cumming. Seismic refra c t i o n studies i n western Canada. Can. J. Earth Sci., 1099-1109, 1972. Crickmay, C.H. The Rocky Mountain Trench: a problem. Can. J. Earth Sci. , 1, 184-205, 1964. Cumming, G.L., and E.R. Kanasewich. Crustal structure i n western Canada. Project Vela Uniform, Final Report, AFCRL-66-159, 1966. Dahlstrom, CD.A. Structural geology of the eastern margin of the Canadian Rocky Mountains. Bull. Can. Soo. Petrol. Geol., 18, 332-406, 1970. Daly, R.A. Geology of the North American C o r d i l l e r a at the 49th p a r a l l e l . Geol. Surv. Can. Memoir 38, Parts 1 and 11, 1912. Daly, R.A. Geological reconnaissance between Golden and Kamloops, B.C. Geol. Surv. Can. Memoir 68, 1915. Dawson, CM. Physical and geological features of the Rocky Mountains. Geol. Surv. Can. Ann. Rept. , 1^, Report B, 1886. 63 Dragert, H. Broad-band geomagnetic depth-sounding along an anomalous p r o f i l e i n the Canadian C o r d i l l e r a . Ph.D. thesis, University of B.C., 1973. Eardley, A.J. Late Cenozoic trenches of the Rocky Mountains. Bull. Geol. Soc. Am., .58, 1176, 1947 (abstract). E l l i s , R.M., and R.D. Russell. Monitoring of seismic a c t i v i t y during loading of Mica Reservoir. Semi-annual technical report to U.S. Geological Survey3 Contract No. 14-08-0001-130673 December, 1972. Evans, C.S. Brisco-Dogtooth Map Area, B.C. Geol. Surv. Can. Summ. Eept. , 1932, AH, 106-176, 1933. Forsyth, D.A.G. A re f r a c t i o n survey across the Canadian C o r d i l l e r a . M.Sc. thesis, University of B.C., 1973. Gabrielse, H. Sedimentary facies and Northern Rocky Mountain Trench. Prog, with Abstr. "Faults^ fractures ^ lineaments and related mineralization in the Canadian Cordillera". Cordilleran Sect. Geol. Assoc. Can. conference, Feb., 1972. Garland, G.D., E.R. Kanasawich, and T.L. Thompson. Gravity measurements over the southern Rocky Mountain Trench area of B.C. J. Geo-phys. Res., 66, 2495-2505, 1961. Haines, G.V., W. Hannaford, and R.P. Riddihough. Magnetic anomalies over B r i t i s h Columbia and the adjacent P a c i f i c Ocean. Can. J. Earth Sci., 8, 387-391, 1971. Hales, A.L., and J.B. Nation. A seismic refr a c t i o n survey i n the Northern Rocky Mountains: More evidence for an intermediate cr u s t a l layer. Contribution No. 228, Inst. Geol. Sciences} University of Texas at Dallas, 1973 (in press). Henderson, G,G.L. A summary of the regional structure and stratigraphy of the Rocky Mountain Trench. Can. Inst. Min. Metal. Bull., 52, 322-327, 1959. Holland, S.S. Symposium on the Rocky Mountain Trench: Introduction. Can. Inst. Min. Metal. Bull., 52, 318, 1959. Jones, H.J., and J.A. Morrison. Cross-correlation f i l t e r i n g . Geophysics, 19, 660-683, 1954. Kanasewich, E.R., R.M. Clowes, and C.H. McCloughan. A buried precambrian r i f t i n Western Canada. Tecton. ,8, 513-527, 1969. Kanasewich, E.R. Time Sequence Ana-lysis jin Geophysics, (pre-publication manuscript), University of Alberta Press, 1974, (in press). 64 K o l l a r , F. and R.D. Russell. Seismometer analysis using an e l e c t r i c current analog. Bull. Seism. Soc. Am., _56, 1193-1205, 1966. Lamb, A.T. and D.W. Smith. Refraction p r o f i l e s over the southern Rocky Mountain Trench area of B.C. J. Alta. Soc. Petrol. Geol., 10, 428-437, 1962. Law, L.K. and R.P. Riddihough. A geographical r e l a t i o n between geomagnetic va r i a t i o n anomalies and tectonics. Can. J. Earth Sci. , j}, 1094-1106, 1971. Leech, G.B. The southern part of the Rocky Mountain Trench. Can. Inst. Min. Metal. Bull. , _5J2, 327-333, 1959. Leech, G.B. Rocky Mountain Trench. Geol. Soc. Am. Spec. Paper 76, p. 100, 1964 (abstract). Leech, G.B. The Rocky Mountain Trench, i n The World R i f t System, Geol. Surv. Can. Paper 66-14, 307-329, 1965. Monger, J.W.H., J.G. Souther and H. Gabrielse. Evolution of the Canadian C o r d i l l e r a : a plate-tectonic model. Am. J. Sci., 272, 577-602, 1972. Mudge, M.R. Origin of the disturbed b e l t i n northwestern Montana. Bull. Geol. Soc. Am., 81, 377-392, 1970. Mueller, S. and M, Landisman. Seismic studies of the earth's crust i n continents. Part I: Evidence f o r a low v e l o c i t y zone i n the upper part of the lithosphere. Geop. J. Roy. Astr. Soc, 10, 525-538, 1966. North, F.K. and G.G.L. Henderson. The Rocky Mountain Trench. Alta. Soc. Petrol. Geol. 4th Annual Field Conference Guide Book, 82-100, 1954. P r i c e , R.A. and E.W. Mountjoy. Geologic structure of the Canadian Rocky Mountains between Bow and Athabasca Rivers. Geol. Assoc. Can. Spec. Paper No. 6, 7-25, 19 70. Roddick, J.A. Tin t i n a Trench. J. Geol., 75, 23-33, 1967. Schofield, S.J. The o r i g i n of the Rocky Mountain Trench, B.C. Trans. Roy. Soc. Can., 3rd ser., 14, sec. 4, 61-97, 1921. Shepard, F.P. The s t r u c t u r a l r e l a t i o n of the Puree 11 Range and the Rocky Mountains of Canada. J. Geol., 30, 130-139, 1922. Shepard, F.P. Further investigations of the Rocky Mountain Trench. J. Geol., 34, 623-641, 1926. Simony, P., H. Baer, E. Ghent, Y. Helfenbein, F. M e i l l i e z and J. Terry. Structural d e t a i l i n a portion of the southern Rocky Mountain Trench. Prog, with Abstr. "Faults, fractures, lineaments and related mineralization in the Canadian Cordillera". Cordilleran Sect. Geol. Assoc. Can. conference, Feb., 1972. 65 Souther, J.G. Volcanism and i t s relationship to recent cr u s t a l movements i n the Canadian C o r d i l l e r a . Can. J. Earth Sci., ]_, 553-568, 1970. Stacey, R.A. Gravity anomalies, crustal structure and plate tectonics i n the Canadian C o r d i l l e r a . Can. J. Earth Sci., 10, 615-628, 1973. Tempelman-Kluit, D.J. Evidence for timing and magnitude of movement along Tin t i n a Trench. Prog, with Abstr. "Faults, fractures, lineaments, and related mineralization in the Canadian Cordillera". C o r d i l l e r a n Sect. Geol. Assoc. Can. conference, Feb., 19721 Thompson, T.L. Origin of the Rocky Mountain Trench i n southern B.C. by Cenozoic block f a u l t i n g . J. Alta. Soc. Petrol. Geol., 10, 408-427, 1962. Walker, J.F. Geology and mineral deposits of Windermere map-area. Geol. Surv. Can. Memoir 148, 1926. White, W.R.H., M.N. Bone and W.G. Milne. Seismic re f r a c t i o n surveys i n B r i t i s h Columbia - a preliminary interpretation. Am. Geophys. Vn. Mono. No. 12, 81-93, 1968. Wiggins, R.A. and J.A. Madrid. Body wave amplitude calculations. submitted to Geop. J., 1974 ( i n press). 66 APPENDIX Corrections for HRGLTZ Fault Models In this procedure the following assumptions have been made: (1) The equations of geometric ray theory are v a l i d for the calculation of traveltimes. (2) Average v e l o c i t i e s can be used i n layers containing v e l o c i t y gradients. (3) The f a u l t throw i s much less than the horizontal distance of the f a u l t from the shot point. (4) Seismic energy i n the body wave incident upon the lower corner of the f a u l t i s p a r t i a l l y converted into a head wave which continues along the downthrown surface. 1. Correction to P intercept time f o r a f a u l t on the basement g h l V ° h i ( h i = h j + h t ) h t Denote the head wave intercept time by T and average v e l o c i t i e s by V. Ti = 2h}/ V j 2 - V 0 2 (VoVi)" 1 for a model based on the up-fault structure Ti = 2h}/ V i 2 - V Q 2 (VQVI) 1 f o r a model based on the down-fault structure I f AT i s the delay time i n the data due to the f a u l t , and AT i s the time correction which must be added to the down-fault intercept i n the data ( i . e . a HRGLTZ model delayed by AT from the down-fault i intercept w i l l give the desired h^ s t r u c t u r e ) , then: AT = Ti - (Ti + AT) By assumptions (3) and (4) i t can be shown that: AT = h tV!(V 0/ V 2 2 - V 0 2 ) 1 67 Therefore, AT = 2(h[ - h x) / V i 2 - V 0 2 (VQVX)" 1 = 2Ax(V^ 2 - V 0 2) ( V i ) " 2 - AT AT = Ax(V! 2 - 2V 0 2)/V 1 2 In the seismic data, - Ax AT = V l = V n = 1.7 sec 6.5 km/s 5.7 km/s -> AT = -0.89 sec The value of the average upper cru s t a l v e l o c i t y was repeatedly checked during the modelling to ensure i t s accuracy. Therefore, a HRGLTZ model was produced such that i t s P^ intercept was e a r l i e r than the down-fault P^ intercept i n the data by 0.89 sec. This gave hi = 12.1 km. Knowing that hi = 6.5 km from the up-fault model, a throw of h = 5.6 km was calculated f o r the basement f a u l t . 2.0 General correction to P intercept time for fa u l t s on the basement n and Moho Vi i h 2 v 2 Let hi and h? be the f a u l t throws at the basement and Moho surfaces, respectively. T = 2 h j / V 2 2 - V 0 2 /V 2V 0 + 2h 2/ V 2 2 - V x 2 /V 2Vj for a model based on the up-fault structure 68 i i _ _ i _ _ T 2 = 2h1V V22 - V 0 2 /V 2V 0 + 2h 2/.V 2 2 " v l 2 /V 2Vi for a model based on the down-fault structure Let ATi be the delay time i n the data due to the action of the basement f a u l t (this i s a second order effect only) and A T 2 be the delay time i n the data due to the action of the Moho f a u l t . Therefore: » AT = T 2 - (T 2 + AT2. + Ax 2) As i n Section 1, i t can be shown that: Ax 2 = h2t.V2 ( V 2 / V 2 2 - V!2 ) _ 1 I t can also be shown that: ATJ = h l t V 2 { ( V 0 / V 2 2 - V 0 2 ) _ 1 - (V 2/ V 2 2 - V j 2 ) _ 1> Therefore, i t follows that: AT = 2hi / V 2 2 - V 0 2 /V 2V 0 + 2(h 2 - h 2 ) / V 2 2 - V j 2 /V 2Vi - h l tV 2{CV 0/ V 2 2 - V 0 2 ) _ 1 - ( V i / V 2 2 - V j 2 ) _ 1 } - h2j.V2 ( V 2 / V 2 2 - V i 2 ) _ 1 This r e s u l t w i l l be applied i n the following sections. 2.1 Faulted basement, unfaulted Moho (Figures 12a and 12b) i ho =0 and ho - ho = -hi t AT = 2 h i t { ( / V 2 2 - V 0 2 /V0) - (/ V 2 2 - V j 2 /V!)}/V 2 - h l t V 2 { ( V 0 / V 2 2 - V 0 2 ) _ 1 - ( V x / V 2 2 - V i 2 ) _ 1 } In the seismic data, V 2 =8.2 km/s Vx =6.8 km/s V x = 6.5 km/s V 0 =5.7 km/s 69 From the calculations i n Section 1, h ^ = 5.6 km. Therefore: AT = 0.54 sec As i n Section 1, the value of the average lower c r u s t a l v e l o c i t y was repeatedly checked during the modelling to ensure i t s accuracy. Therefore, a HRGLTZ model was produced such that i t s P^ intercept was delayed from the P intercept i n the data by 0.54 sec. The re s u l t i n g model gives a cr u s t a l thickness of (hi + h 2) = 61.2 km before and after the basement f a u l t . 2.2 Constant f a u l t throw throughout the cru s t a l section (Figures 13a and 13b) n i t = h 2 and h 2 - h 2 = 0 AT = 2 h l t ( / V 2 2 - V 0 2 /V 0 )/V 2 - h i t V 2 { ( V 0 / V 2 2 - V 0 2 )~ l - (V] / V 2 2 - V i 2 ) 1 + ( V i / v 2 2 - V x 2 ) l } For the same data parameters as Section 2.1: AT = -0.01 sec Therefore, a HRGLTZ model was produced such that i t s P^ intercept was e a r l i e r than the P intercept i n the data by 0.01 sec. The resu l t i n g n i i model gives a down-fault crustal thickness of Chi + h 2) = 57.8 km. Since the f a u l t throw i s assumed to be constant throught the cr u s t a l section, the up-fault c r u s t a l thickness i s then (hi + h 2 - hi ) = 52.2 km. 2.3 Fault throw calculated from the pre-fault P^ intercept i h 2 - h 2 = h 2 - hj and Ax 2 i s picked from the data AT - 2 h l t { ( / V 2 2 - V 0 2 /V0) - (/ V 2 2 - V 2 2 /V!)}/V 2 - h 1 ( ; V 2 { ( V 0 / V 2 2 - V 0 2 ) 1 - ( V i / V 2 2 - V i 2 ) J } - A T 2 { ( 2 V I 2 / V 2 2 ) - 1} 70 Using the same data parameters as Section 2.1, and i n addition: A T 2 = 1.5 sec AT = -0.04 sec Therefore, a HRGLTZ model was produced such that i t s intercept was e a r l i e r than the P intercept i n the data by 0.04 sec. The resu l t i n g n i i structure gave a down-fault c r u s t a l thickness (hj + h 2 ) =57.6 km. A HRGLTZ model was then run for the up-fault model such that i t s P n intercept was delayed from the P^ intercept i n the data (the up-fault intercept given by the phase on C13 and B4) by an amount AT^ due to the extra thickness of the down-fault upper crustal layer. From the above calcu l a t i o n s , ATJ= -0.05 sec, therefore the HRGLTZ intercept was early by 0.05 sec. The resu l t i n g model gave an upfault crustal thickness of (hj + h 2) = 50.8 km. Therefore, the f a u l t throw on the Moho i s ho =6.8 km. z t 

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